[ Upstream commit 9c0e343d7654a329d1f9b53d253cbf7fb6eff85d ]
We should get psr value from regs->psr in stack, not directly get
it from phyiscal register then save the vector number in
tsk->trap_no.
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 359ae00d12589c31cf103894d0f32588d523ca83 ]
During ftrace init, linux will replace all function prologues
(call_mcout) with nops, but it need flush_dcache and
invalidate_icache to make it work. So flush_cache functions
couldn't be nested called by ftrace framework.
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 7f4a567332f035ab16b29010fbd04a0f10183c77 ]
There is no present bit in csky pmd hardware, so we need to prepare invalid_pte_table
for empty pmd entry and the functions (pmd_none & pmd_present) in pgtable.h need
invalid_pte_talbe to get result. If a module use these functions, we need export the
symbol for it.
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Mo Qihui <qihui.mo@verisilicon.com>
Cc: Zhange Jian <zhang_jian5@dahuatech.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
The csky implementation of free_initrd_mem() is an open-coded version of
free_reserved_area() without poisoning.
Remove it and make csky use the generic version of free_initrd_mem().
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
The memory allocated for the atomic pool needs to have the same
mapping attributes that we use for remapping, so use
pgprot_dmacoherent instead of open coding it. Also deduct a
suitable zone to allocate the memory from based on the presence
of the DMA zones.
Signed-off-by: Christoph Hellwig <hch@lst.de>
DMA_FROM_DEVICE only need to read dma data of memory into CPU cache,
so there is no need to clear cache before. Also clear + inv for
DMA_FROM_DEVICE won't cause problem, because the memory range for dma
won't be touched by software during dma working.
Changes for V2:
- Remove clr cache and ignore the DMA_TO_DEVICE in _for_cpu.
- Change inv to wbinv cache with DMA_FROM_DEVICE in _for_device.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
If the paddr and size are cross between NORMAL_ZONE and HIGHMEM_ZONE
memory range, cache_op will panic in do_page_fault with bad_area.
Optimize the code to support the range which cross memory ZONEs.
Changes for V2:
- Revert back to postcore_initcall
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
This bug is from commit: 2b070ccdf8 (fixup abiv2 mmap(... O_SYNC)
failed). In that patch we remove the _PAGE_SO for memory noncache
mapping and this will cause problem when drivers use dma descriptors
to control the transcations without dma_w/rmb().
After referencing other archs' implementation, pgprot_writecombine is
introduced for mmap(... O_SYNC).
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
This round of csky subsystem gives two features (ASID algorithm update,
Perf pmu record support) and some fixups.
Feature:
- csky: Revert mmu ASID mechanism
- csky: Add new asid lib code from arm
- csky: Use generic asid algorithm to implement switch_mm
- csky: Improve tlb operation with help of asid
- csky: Init pmu as a device
- csky: Add count-width property for csky pmu
- csky: Add pmu interrupt support
- csky: Fix perf record in kernel/user space
- dt-bindings: csky: Add csky PMU bindings
Fixup:
- csky: Fixup no panic in kernel for some traps
- csky: Fixup some error count in 810 & 860.
- csky: Fixup abiv1 memset error
CI-Tested: https://gitlab.com/c-sky/buildroot/pipelines/68656845
-----BEGIN PGP SIGNATURE-----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=PbF1
-----END PGP SIGNATURE-----
Merge tag 'csky-for-linus-5.3-rc1' of git://github.com/c-sky/csky-linux
Pull arch/csky pupdates from Guo Ren:
"This round of csky subsystem gives two features (ASID algorithm
update, Perf pmu record support) and some fixups.
ASID updates:
- Revert mmu ASID mechanism
- Add new asid lib code from arm
- Use generic asid algorithm to implement switch_mm
- Improve tlb operation with help of asid
Perf pmu record support:
- Init pmu as a device
- Add count-width property for csky pmu
- Add pmu interrupt support
- Fix perf record in kernel/user space
- dt-bindings: Add csky PMU bindings
Fixes:
- Fixup no panic in kernel for some traps
- Fixup some error count in 810 & 860.
- Fixup abiv1 memset error"
* tag 'csky-for-linus-5.3-rc1' of git://github.com/c-sky/csky-linux:
csky: Fixup abiv1 memset error
csky: Improve tlb operation with help of asid
csky: Use generic asid algorithm to implement switch_mm
csky: Add new asid lib code from arm
csky: Revert mmu ASID mechanism
dt-bindings: csky: Add csky PMU bindings
dt-bindings: interrupt-controller: Update csky mpintc
csky: Fixup some error count in 810 & 860.
csky: Fix perf record in kernel/user space
csky: Add pmu interrupt support
csky: Add count-width property for csky pmu
csky: Init pmu as a device
csky: Fixup no panic in kernel for some traps
csky: Select intc & timer drivers
There are two generations of tlb operation instruction for C-SKY.
First generation is use mcr register and it need software do more
things, second generation is use specific instructions, eg:
tlbi.va, tlbi.vas, tlbi.alls
We implemented the following functions:
- flush_tlb_range (a range of entries)
- flush_tlb_page (one entry)
Above functions use asid from vma->mm to invalid tlb entries and
we could use tlbi.vas instruction for newest generation csky cpu.
- flush_tlb_kernel_range
- flush_tlb_one
Above functions don't care asid and it invalid the tlb entries only
with vpn and we could use tlbi.vaas instruction for newest generat-
ion csky cpu.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Use linux generic asid/vmid algorithm to implement csky
switch_mm function. The algorithm is from arm and it could
work with SMP system. It'll help reduce tlb flush for
switch_mm in task/vm switch.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
This patch only contains asid help code from arm for next patch to
use.
The asid allocator use five level check to reduce the cost of
switch_mm.
1. Check if the asid version is the same (it's general)
2. Check reserved_asid which is set in rollover flush_context()
and key point is to keep the same bit position with the current
asid version instead of input version.
3. Check if the position of bitmap is free then it could be set &
used directly.
4. find_next_zero_bit() (a little performance cost)
5. flush_context (this is the worst cost with increase current asid
version)
Check is level by level and cost is also higher with the next level.
The reserved_asid and bitmap mechanism prevent unnecessary
find_next_zero_bit().
The atomic 64 bit asid is also suitable for 32-bit system and it
won't cost a lot in 1th 2th 3th level check.
The operation of set/clear mm_cpumask was removed in arm64 compared to
arm32. It seems no side effect on current arm64 system, but from
software meaning it's wrong. Although csky also needn't it, we add it
back for csky.
The asid_per_ctxt is no use for csky and it reserves the lowest bits for
other use, maybe: trust zone ? Ok, just keep it in csky copy.
Seems it also could be used by other archs and it's worth to move asid
code to generic in future.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Julien Grall <julien.grall@arm.com>
Current C-SKY ASID mechanism is from mips and it doesn't work well
with multi-cores. ASID per core mechanism is not suitable for C-SKY
SMP tlb maintain operations, eg: tlbi.vas need share the same asid
in all processors and it'll invalid the tlb entry in all cores with
the same asid.
This patch is prepare for new ASID mechanism.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Pull force_sig() argument change from Eric Biederman:
"A source of error over the years has been that force_sig has taken a
task parameter when it is only safe to use force_sig with the current
task.
The force_sig function is built for delivering synchronous signals
such as SIGSEGV where the userspace application caused a synchronous
fault (such as a page fault) and the kernel responded with a signal.
Because the name force_sig does not make this clear, and because the
force_sig takes a task parameter the function force_sig has been
abused for sending other kinds of signals over the years. Slowly those
have been fixed when the oopses have been tracked down.
This set of changes fixes the remaining abusers of force_sig and
carefully rips out the task parameter from force_sig and friends
making this kind of error almost impossible in the future"
* 'siginfo-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace: (27 commits)
signal/x86: Move tsk inside of CONFIG_MEMORY_FAILURE in do_sigbus
signal: Remove the signal number and task parameters from force_sig_info
signal: Factor force_sig_info_to_task out of force_sig_info
signal: Generate the siginfo in force_sig
signal: Move the computation of force into send_signal and correct it.
signal: Properly set TRACE_SIGNAL_LOSE_INFO in __send_signal
signal: Remove the task parameter from force_sig_fault
signal: Use force_sig_fault_to_task for the two calls that don't deliver to current
signal: Explicitly call force_sig_fault on current
signal/unicore32: Remove tsk parameter from __do_user_fault
signal/arm: Remove tsk parameter from __do_user_fault
signal/arm: Remove tsk parameter from ptrace_break
signal/nds32: Remove tsk parameter from send_sigtrap
signal/riscv: Remove tsk parameter from do_trap
signal/sh: Remove tsk parameter from force_sig_info_fault
signal/um: Remove task parameter from send_sigtrap
signal/x86: Remove task parameter from send_sigtrap
signal: Remove task parameter from force_sig_mceerr
signal: Remove task parameter from force_sig
signal: Remove task parameter from force_sigsegv
...
As synchronous exceptions really only make sense against the current
task (otherwise how are you synchronous) remove the task parameter
from from force_sig_fault to make it explicit that is what is going
on.
The two known exceptions that deliver a synchronous exception to a
stopped ptraced task have already been changed to
force_sig_fault_to_task.
The callers have been changed with the following emacs regular expression
(with obvious variations on the architectures that take more arguments)
to avoid typos:
force_sig_fault[(]\([^,]+\)[,]\([^,]+\)[,]\([^,]+\)[,]\W+current[)]
->
force_sig_fault(\1,\2,\3)
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Add SPDX license identifiers to all Make/Kconfig files which:
- Have no license information of any form
These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:
GPL-2.0-only
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
The function of __va() will return "void *", but the pgd_base is
unsigned long.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
This patch add support for page fault count, major fault count
and minorfault count. Without this patch page faults are not
sampled for perf event.
Performance counter stats for '/usr/lib/perf-test/callchain_test':
0 page-faults # 0.000 K/sec
Signed-off-by: Mao Han <han_mao@c-sky.com>
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
The name of phys_offset is so common for global export and it may
conflict with some local name. So change phys_offset to va_pa_offset
which also used by riscv.
Also use __pa() and __va() instead of using phys_offset directly.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Add check for the return value of memblock_alloc*() functions and call
panic() in case of error. The panic message repeats the one used by
panicing memblock allocators with adjustment of parameters to include
only relevant ones.
The replacement was mostly automated with semantic patches like the one
below with manual massaging of format strings.
@@
expression ptr, size, align;
@@
ptr = memblock_alloc(size, align);
+ if (!ptr)
+ panic("%s: Failed to allocate %lu bytes align=0x%lx\n", __func__, size, align);
[anders.roxell@linaro.org: use '%pa' with 'phys_addr_t' type]
Link: http://lkml.kernel.org/r/20190131161046.21886-1-anders.roxell@linaro.org
[rppt@linux.ibm.com: fix format strings for panics after memblock_alloc]
Link: http://lkml.kernel.org/r/1548950940-15145-1-git-send-email-rppt@linux.ibm.com
[rppt@linux.ibm.com: don't panic if the allocation in sparse_buffer_init fails]
Link: http://lkml.kernel.org/r/20190131074018.GD28876@rapoport-lnx
[akpm@linux-foundation.org: fix xtensa printk warning]
Link: http://lkml.kernel.org/r/1548057848-15136-20-git-send-email-rppt@linux.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Guo Ren <ren_guo@c-sky.com> [c-sky]
Acked-by: Paul Burton <paul.burton@mips.com> [MIPS]
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com> [s390]
Reviewed-by: Juergen Gross <jgross@suse.com> [Xen]
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org> [m68k]
Acked-by: Max Filippov <jcmvbkbc@gmail.com> [xtensa]
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Greentime Hu <green.hu@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guan Xuetao <gxt@pku.edu.cn>
Cc: Guo Ren <guoren@kernel.org>
Cc: Mark Salter <msalter@redhat.com>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Rich Felker <dalias@libc.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Stafford Horne <shorne@gmail.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Some user space drivers need accessing IO address and IO remap need
SO(strong order) page-attribute to make IO operation correct. So we
need add SO-page-attr for all non-memory address.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reported-by: Fan Xiaodong <xiaodong.fan@boyahualu.com>
Cleanup struct cpuinfo_csky and struct thread_struct, remove all esp0
related code. We could get pt_regs from sp and backtrace could use fp
in switch_stack.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Glibc function mmap(... O_SYNC) will make page to _PAGE_UNCACHE +
_PAGE_SO and strong-order page couldn't support unalignment access.
So remove _PAGE_SO from _PAGE_UNCACHE, also sync abiv1 with the macro
of _PAGE_SO.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reported-by: Liu Renwei <Renwei.Liu@verisilicon.com>
Tested-by: Yuan Qiyun <qiyun_yuan@c-sky.com>
Merge misc updates from Andrew Morton:
- large KASAN update to use arm's "software tag-based mode"
- a few misc things
- sh updates
- ocfs2 updates
- just about all of MM
* emailed patches from Andrew Morton <akpm@linux-foundation.org>: (167 commits)
kernel/fork.c: mark 'stack_vm_area' with __maybe_unused
memcg, oom: notify on oom killer invocation from the charge path
mm, swap: fix swapoff with KSM pages
include/linux/gfp.h: fix typo
mm/hmm: fix memremap.h, move dev_page_fault_t callback to hmm
hugetlbfs: Use i_mmap_rwsem to fix page fault/truncate race
hugetlbfs: use i_mmap_rwsem for more pmd sharing synchronization
memory_hotplug: add missing newlines to debugging output
mm: remove __hugepage_set_anon_rmap()
include/linux/vmstat.h: remove unused page state adjustment macro
mm/page_alloc.c: allow error injection
mm: migrate: drop unused argument of migrate_page_move_mapping()
blkdev: avoid migration stalls for blkdev pages
mm: migrate: provide buffer_migrate_page_norefs()
mm: migrate: move migrate_page_lock_buffers()
mm: migrate: lock buffers before migrate_page_move_mapping()
mm: migration: factor out code to compute expected number of page references
mm, page_alloc: enable pcpu_drain with zone capability
kmemleak: add config to select auto scan
mm/page_alloc.c: don't call kasan_free_pages() at deferred mem init
...
totalram_pages and totalhigh_pages are made static inline function.
Main motivation was that managed_page_count_lock handling was complicating
things. It was discussed in length here,
https://lore.kernel.org/patchwork/patch/995739/#1181785 So it seemes
better to remove the lock and convert variables to atomic, with preventing
poteintial store-to-read tearing as a bonus.
[akpm@linux-foundation.org: coding style fixes]
Link: http://lkml.kernel.org/r/1542090790-21750-4-git-send-email-arunks@codeaurora.org
Signed-off-by: Arun KS <arunks@codeaurora.org>
Suggested-by: Michal Hocko <mhocko@suse.com>
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
The csky code was largely copied from arm/arm64, so switch to the
generic arm64-based implementation instead.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Guo Ren <ren_guo@c-sky.com>
csky does not implement ZONE_DMA, which means passing GFP_DMA is a no-op.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Guo Ren <ren_guo@c-sky.com>
All architecures use memblock for early memory management. There is no need
for the CONFIG_HAVE_MEMBLOCK configuration option.
[rppt@linux.vnet.ibm.com: of/fdt: fixup #ifdefs]
Link: http://lkml.kernel.org/r/20180919103457.GA20545@rapoport-lnx
[rppt@linux.vnet.ibm.com: csky: fixups after bootmem removal]
Link: http://lkml.kernel.org/r/20180926112744.GC4628@rapoport-lnx
[rppt@linux.vnet.ibm.com: remove stale #else and the code it protects]
Link: http://lkml.kernel.org/r/1538067825-24835-1-git-send-email-rppt@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1536927045-23536-4-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Tested-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greentime Hu <green.hu@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guan Xuetao <gxt@pku.edu.cn>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ley Foon Tan <lftan@altera.com>
Cc: Mark Salter <msalter@redhat.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Rich Felker <dalias@libc.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Serge Semin <fancer.lancer@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This patch adds files related to memory management and here is our
memory-layout:
Fixmap : 0xffc02000 – 0xfffff000 (4 MB - 12KB)
Pkmap : 0xff800000 – 0xffc00000 (4 MB)
Vmalloc : 0xf0200000 – 0xff000000 (238 MB)
Lowmem : 0x80000000 – 0xc0000000 (1GB)
abiv1 CPU (CK610) is VIPT cache and it doesn't support highmem.
abiv2 CPUs are all PIPT cache and they could support highmem.
Lowmem is directly mapped by msa0 & msa1 reg, and we needn't setup
memory page table for it.
Link:https://lore.kernel.org/lkml/20180518215548.GH17671@n2100.armlinux.org.uk/
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
