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KVM: PPC: Book3S HV: Snapshot timebase offset on guest entry 

Currently, the HV KVM guest entry/exit code adds the timebase offset
from the vcore struct to the timebase on guest entry, and subtracts
it on guest exit.  Which is fine, except that it is possible for
userspace to change the offset using the SET_ONE_REG interface while
the vcore is running, as there is only one timebase offset per vcore
but potentially multiple VCPUs in the vcore.  If that were to happen,
KVM would subtract a different offset on guest exit from that which
it had added on guest entry, leading to the timebase being out of sync
between cores in the host, which then leads to bad things happening
such as hangs and spurious watchdog timeouts.

To fix this, we add a new field 'tb_offset_applied' to the vcore struct
which stores the offset that is currently applied to the timebase.
This value is set from the vcore tb_offset field on guest entry, and
is what is subtracted from the timebase on guest exit.  Since it is
zero when the timebase offset is not applied, we can simplify the
logic in kvmhv_start_timing and kvmhv_accumulate_time.

In addition, we had secondary threads reading the timebase while
running concurrently with code on the primary thread which would
eventually add or subtract the timebase offset from the timebase.
This occurred while saving or restoring the DEC register value on
the secondary threads.  Although no specific incorrect behaviour has
been observed, this is a race which should be fixed.  To fix it, we
move the DEC saving code to just before we call kvmhv_commence_exit,
and the DEC restoring code to after the point where we have waited
for the primary thread to switch the MMU context and add the timebase
offset.  That way we are sure that the timebase contains the guest
timebase value in both cases.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This commit is contained in:
Paul Mackerras 2018-04-20 22:51:11 +10:00
parent 75bc37fefc
commit 57b8daa70a
4 changed files with 47 additions and 45 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
arch/powerpc/include/asm/kvm_book3s.h
View File

@ -96,6 +96,7 @@ struct kvmppc_vcore {
struct kvm_vcpu *runner;
struct kvm *kvm;
u64 tb_offset; /* guest timebase - host timebase */
u64 tb_offset_applied; /* timebase offset currently in force */
ulong lpcr;
u32 arch_compat;
ulong pcr;

1
arch/powerpc/kernel/asm-offsets.c
View File

@ -562,6 +562,7 @@ int main(void)
OFFSET(VCORE_NAPPING_THREADS, kvmppc_vcore, napping_threads);
OFFSET(VCORE_KVM, kvmppc_vcore, kvm);
OFFSET(VCORE_TB_OFFSET, kvmppc_vcore, tb_offset);
OFFSET(VCORE_TB_OFFSET_APPL, kvmppc_vcore, tb_offset_applied);
OFFSET(VCORE_LPCR, kvmppc_vcore, lpcr);
OFFSET(VCORE_PCR, kvmppc_vcore, pcr);
OFFSET(VCORE_DPDES, kvmppc_vcore, dpdes);

1
arch/powerpc/kvm/book3s_hv.c
View File

@ -2441,6 +2441,7 @@ static void init_vcore_to_run(struct kvmppc_vcore *vc)
vc->in_guest = 0;
vc->napping_threads = 0;
vc->conferring_threads = 0;
vc->tb_offset_applied = 0;
}
static bool can_dynamic_split(struct kvmppc_vcore *vc, struct core_info *cip)

89
arch/powerpc/kvm/book3s_hv_rmhandlers.S
View File

@ -692,6 +692,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
22: ld r8,VCORE_TB_OFFSET(r5)
cmpdi r8,0
beq 37f
std r8, VCORE_TB_OFFSET_APPL(r5)
mftb r6 /* current host timebase */
add r8,r8,r6
mtspr SPRN_TBU40,r8 /* update upper 40 bits */
@ -940,18 +941,6 @@ FTR_SECTION_ELSE
ALT_FTR_SECTION_END_IFCLR(CPU_FTR_ARCH_300)
8:
/*
* Set the decrementer to the guest decrementer.
*/
ld r8,VCPU_DEC_EXPIRES(r4)
/* r8 is a host timebase value here, convert to guest TB */
ld r5,HSTATE_KVM_VCORE(r13)
ld r6,VCORE_TB_OFFSET(r5)
add r8,r8,r6
mftb r7
subf r3,r7,r8
mtspr SPRN_DEC,r3
ld r5, VCPU_SPRG0(r4)
ld r6, VCPU_SPRG1(r4)
ld r7, VCPU_SPRG2(r4)
@ -1005,6 +994,18 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_ARCH_300)
mtspr SPRN_LPCR,r8
isync
/*
* Set the decrementer to the guest decrementer.
*/
ld r8,VCPU_DEC_EXPIRES(r4)
/* r8 is a host timebase value here, convert to guest TB */
ld r5,HSTATE_KVM_VCORE(r13)
ld r6,VCORE_TB_OFFSET_APPL(r5)
add r8,r8,r6
mftb r7
subf r3,r7,r8
mtspr SPRN_DEC,r3
/* Check if HDEC expires soon */
mfspr r3, SPRN_HDEC
EXTEND_HDEC(r3)
@ -1597,8 +1598,27 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
guest_bypass:
stw r12, STACK_SLOT_TRAP(r1)
mr r3, r12
/* Save DEC */
/* Do this before kvmhv_commence_exit so we know TB is guest TB */
ld r3, HSTATE_KVM_VCORE(r13)
mfspr r5,SPRN_DEC
mftb r6
/* On P9, if the guest has large decr enabled, don't sign extend */
BEGIN_FTR_SECTION
ld r4, VCORE_LPCR(r3)
andis. r4, r4, LPCR_LD@h
bne 16f
END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
extsw r5,r5
16: add r5,r5,r6
/* r5 is a guest timebase value here, convert to host TB */
ld r4,VCORE_TB_OFFSET_APPL(r3)
subf r5,r4,r5
std r5,VCPU_DEC_EXPIRES(r9)
/* Increment exit count, poke other threads to exit */
mr r3, r12
bl kvmhv_commence_exit
nop
ld r9, HSTATE_KVM_VCPU(r13)
@ -1639,23 +1659,6 @@ guest_bypass:
mtspr SPRN_PURR,r3
mtspr SPRN_SPURR,r4
/* Save DEC */
ld r3, HSTATE_KVM_VCORE(r13)
mfspr r5,SPRN_DEC
mftb r6
/* On P9, if the guest has large decr enabled, don't sign extend */
BEGIN_FTR_SECTION
ld r4, VCORE_LPCR(r3)
andis. r4, r4, LPCR_LD@h
bne 16f
END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
extsw r5,r5
16: add r5,r5,r6
/* r5 is a guest timebase value here, convert to host TB */
ld r4,VCORE_TB_OFFSET(r3)
subf r5,r4,r5
std r5,VCPU_DEC_EXPIRES(r9)
BEGIN_FTR_SECTION
b 8f
END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
@ -2017,9 +2020,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
27:
/* Subtract timebase offset from timebase */
ld r8,VCORE_TB_OFFSET(r5)
ld r8, VCORE_TB_OFFSET_APPL(r5)
cmpdi r8,0
beq 17f
li r0, 0
std r0, VCORE_TB_OFFSET_APPL(r5)
mftb r6 /* current guest timebase */
subf r8,r8,r6
mtspr SPRN_TBU40,r8 /* update upper 40 bits */
@ -2700,7 +2705,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
add r3, r3, r5
ld r4, HSTATE_KVM_VCPU(r13)
ld r5, HSTATE_KVM_VCORE(r13)
ld r6, VCORE_TB_OFFSET(r5)
ld r6, VCORE_TB_OFFSET_APPL(r5)
subf r3, r6, r3 /* convert to host TB value */
std r3, VCPU_DEC_EXPIRES(r4)
@ -2799,7 +2804,7 @@ END_FTR_SECTION(CPU_FTR_TM | CPU_FTR_P9_TM_HV_ASSIST, 0)
/* Restore guest decrementer */
ld r3, VCPU_DEC_EXPIRES(r4)
ld r5, HSTATE_KVM_VCORE(r13)
ld r6, VCORE_TB_OFFSET(r5)
ld r6, VCORE_TB_OFFSET_APPL(r5)
add r3, r3, r6 /* convert host TB to guest TB value */
mftb r7
subf r3, r7, r3
@ -3606,12 +3611,9 @@ kvmppc_fix_pmao:
*/
kvmhv_start_timing:
ld r5, HSTATE_KVM_VCORE(r13)
lbz r6, VCORE_IN_GUEST(r5)
cmpwi r6, 0
beq 5f /* if in guest, need to */
ld r6, VCORE_TB_OFFSET(r5) /* subtract timebase offset */
5: mftb r5
subf r5, r6, r5
ld r6, VCORE_TB_OFFSET_APPL(r5)
mftb r5
subf r5, r6, r5 /* subtract current timebase offset */
std r3, VCPU_CUR_ACTIVITY(r4)
std r5, VCPU_ACTIVITY_START(r4)
blr
@ -3622,15 +3624,12 @@ kvmhv_start_timing:
*/
kvmhv_accumulate_time:
ld r5, HSTATE_KVM_VCORE(r13)
lbz r8, VCORE_IN_GUEST(r5)
cmpwi r8, 0
beq 4f /* if in guest, need to */
ld r8, VCORE_TB_OFFSET(r5) /* subtract timebase offset */
4: ld r5, VCPU_CUR_ACTIVITY(r4)
ld r8, VCORE_TB_OFFSET_APPL(r5)
ld r5, VCPU_CUR_ACTIVITY(r4)
ld r6, VCPU_ACTIVITY_START(r4)
std r3, VCPU_CUR_ACTIVITY(r4)
mftb r7
subf r7, r8, r7
subf r7, r8, r7 /* subtract current timebase offset */
std r7, VCPU_ACTIVITY_START(r4)
cmpdi r5, 0
beqlr