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authorPaul Mackerras <paulus@ozlabs.org>2018-04-20 22:51:11 +1000
committerPaul Mackerras <paulus@ozlabs.org>2018-05-17 15:16:45 +1000
commit57b8daa70a179bc23cc4240420ab6fbcdd7faf77 (patch)
tree403fd910b52ba9eac04d286df04db9e7b79b76fb /arch/powerpc/include/asm/kvm_book3s.h
parent75bc37fefc4471e718ba8e651aa74673d4e0a9eb (diff)
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>
Diffstat (limited to 'arch/powerpc/include/asm/kvm_book3s.h')
-rw-r--r--arch/powerpc/include/asm/kvm_book3s.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
index 4c02a7378d06..e7377b73cfec 100644
--- a/arch/powerpc/include/asm/kvm_book3s.h
+++ b/arch/powerpc/include/asm/kvm_book3s.h
@@ -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;