* [PATCH] KVM: X86: always reset st->preempted in record_steal_time()
@ 2021-05-27 17:33 Lai Jiangshan
2021-05-28 6:12 ` Wanpeng Li
0 siblings, 1 reply; 4+ messages in thread
From: Lai Jiangshan @ 2021-05-27 17:33 UTC (permalink / raw)
To: linux-kernel
Cc: Lai Jiangshan, Oliver Upton, Paolo Bonzini, Sean Christopherson,
Vitaly Kuznetsov, Wanpeng Li, Jim Mattson, Joerg Roedel,
Thomas Gleixner, Ingo Molnar, Borislav Petkov, x86,
H. Peter Anvin, Peter Shier, kvm
From: Lai Jiangshan <laijs@linux.alibaba.com>
st->preempted needs to be reset in record_steal_time() to clear the
KVM_VCPU_PREEMPTED bit.
But the commit 66570e966dd9 ("kvm: x86: only provide PV features if
enabled in guest's CPUID") made it cleared conditionally and
KVM_VCPU_PREEMPTED might not be cleared when entering into the guest.
Also make st->preempted be only read once, so that trace_kvm_pv_tlb_flush()
and kvm_vcpu_flush_tlb_guest() is consistent with same value of st->preempted.
Cc: Oliver Upton <oupton@google.com>
Fixes: 66570e966dd9 ("kvm: x86: only provide PV features if enabled in guest's CPUID")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
arch/x86/kvm/x86.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bbc4e04e67ad..b8a7259ebd14 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3101,10 +3101,14 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
* expensive IPIs.
*/
if (guest_pv_has(vcpu, KVM_FEATURE_PV_TLB_FLUSH)) {
+ u8 st_preempted = xchg(&st->preempted, 0);
+
trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
- st->preempted & KVM_VCPU_FLUSH_TLB);
- if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
+ st_preempted & KVM_VCPU_FLUSH_TLB);
+ if (st_preempted & KVM_VCPU_FLUSH_TLB)
kvm_vcpu_flush_tlb_guest(vcpu);
+ } else {
+ st->preempted = 0;
}
vcpu->arch.st.preempted = 0;
--
2.19.1.6.gb485710b
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH] KVM: X86: always reset st->preempted in record_steal_time()
2021-05-27 17:33 [PATCH] KVM: X86: always reset st->preempted in record_steal_time() Lai Jiangshan
@ 2021-05-28 6:12 ` Wanpeng Li
2021-05-31 17:46 ` [PATCH] KVM: X86: reset and read st->preempted in atomic way Lai Jiangshan
0 siblings, 1 reply; 4+ messages in thread
From: Wanpeng Li @ 2021-05-28 6:12 UTC (permalink / raw)
To: Lai Jiangshan
Cc: LKML, Lai Jiangshan, Oliver Upton, Paolo Bonzini,
Sean Christopherson, Vitaly Kuznetsov, Wanpeng Li, Jim Mattson,
Joerg Roedel, Thomas Gleixner, Ingo Molnar, Borislav Petkov,
the arch/x86 maintainers, H. Peter Anvin, Peter Shier, kvm
On Fri, 28 May 2021 at 14:11, Lai Jiangshan <jiangshanlai@gmail.com> wrote:
>
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> st->preempted needs to be reset in record_steal_time() to clear the
> KVM_VCPU_PREEMPTED bit.
>
> But the commit 66570e966dd9 ("kvm: x86: only provide PV features if
> enabled in guest's CPUID") made it cleared conditionally and
> KVM_VCPU_PREEMPTED might not be cleared when entering into the guest.
>
> Also make st->preempted be only read once, so that trace_kvm_pv_tlb_flush()
> and kvm_vcpu_flush_tlb_guest() is consistent with same value of st->preempted.
>
> Cc: Oliver Upton <oupton@google.com>
> Fixes: 66570e966dd9 ("kvm: x86: only provide PV features if enabled in guest's CPUID")
It has already been fixed by commit 1eff0ada88b48 (KVM: X86: Fix vCPU
preempted state from guest's point of view) in kvm/master.
Wanpeng
^ permalink raw reply [flat|nested] 4+ messages in thread
* [PATCH] KVM: X86: reset and read st->preempted in atomic way
2021-05-28 6:12 ` Wanpeng Li
@ 2021-05-31 17:46 ` Lai Jiangshan
2021-06-08 0:33 ` Sean Christopherson
0 siblings, 1 reply; 4+ messages in thread
From: Lai Jiangshan @ 2021-05-31 17:46 UTC (permalink / raw)
To: linux-kernel
Cc: Lai Jiangshan, Paolo Bonzini, Sean Christopherson,
Vitaly Kuznetsov, Wanpeng Li, Jim Mattson, Joerg Roedel,
Thomas Gleixner, Ingo Molnar, Borislav Petkov, x86,
H. Peter Anvin, kvm
From: Lai Jiangshan <laijs@linux.alibaba.com>
In record_steal_time(), st->preempted is read twice, and
trace_kvm_pv_tlb_flush() might output result inconsistent if
kvm_vcpu_flush_tlb_guest() see a different st->preempted later.
It is a very trivial problem and hardly has actual harm and can be
avoided by reseting and reading st->preempted in atomic way via xchg().
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0087d3532c98..fba39fe162da 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3117,9 +3117,11 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
* expensive IPIs.
*/
if (guest_pv_has(vcpu, KVM_FEATURE_PV_TLB_FLUSH)) {
+ u8 st_preempted = xchg(&st->preempted, 0);
+
trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
- st->preempted & KVM_VCPU_FLUSH_TLB);
- if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
+ st_preempted & KVM_VCPU_FLUSH_TLB);
+ if (st_preempted & KVM_VCPU_FLUSH_TLB)
kvm_vcpu_flush_tlb_guest(vcpu);
} else {
st->preempted = 0;
--
2.19.1.6.gb485710b
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH] KVM: X86: reset and read st->preempted in atomic way
2021-05-31 17:46 ` [PATCH] KVM: X86: reset and read st->preempted in atomic way Lai Jiangshan
@ 2021-06-08 0:33 ` Sean Christopherson
0 siblings, 0 replies; 4+ messages in thread
From: Sean Christopherson @ 2021-06-08 0:33 UTC (permalink / raw)
To: Lai Jiangshan
Cc: linux-kernel, Lai Jiangshan, Paolo Bonzini, Vitaly Kuznetsov,
Wanpeng Li, Jim Mattson, Joerg Roedel, Thomas Gleixner,
Ingo Molnar, Borislav Petkov, x86, H. Peter Anvin, kvm
Nit: the shortlog is somewhat inaccurate now, maybe just:
KVM: x86: Ensure PV TLB flush tracepoint reflects KVM behavior
or something along those lines. Not sure what the best wording is :-/
On Tue, Jun 01, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> In record_steal_time(), st->preempted is read twice, and
> trace_kvm_pv_tlb_flush() might output result inconsistent if
> kvm_vcpu_flush_tlb_guest() see a different st->preempted later.
>
> It is a very trivial problem and hardly has actual harm and can be
> avoided by reseting and reading st->preempted in atomic way via xchg().
>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
I saw this quirk too, but couldn't quite bring myself to care enought to test a
patch :-)
Reviewed-by: Sean Christopherson <seanjc@google.com>
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-06-08 0:34 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2021-05-27 17:33 [PATCH] KVM: X86: always reset st->preempted in record_steal_time() Lai Jiangshan
2021-05-28 6:12 ` Wanpeng Li
2021-05-31 17:46 ` [PATCH] KVM: X86: reset and read st->preempted in atomic way Lai Jiangshan
2021-06-08 0:33 ` Sean Christopherson
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