2021-05-28 06:12:08

by Lai Jiangshan

Subject: [PATCH] KVM: X86: always reset st->preempted in record_steal_time()

From: Lai Jiangshan <[email protected]>

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 <[email protected]>
Fixes: 66570e966dd9 ("kvm: x86: only provide PV features if enabled in guest's CPUID")
Signed-off-by: Lai Jiangshan <[email protected]>
---
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

2021-05-28 08:58:00

by Wanpeng Li

Subject: Re: [PATCH] KVM: X86: always reset st->preempted in record_steal_time()

On Fri, 28 May 2021 at 14:11, Lai Jiangshan <[email protected]> wrote:
>
> From: Lai Jiangshan <[email protected]>
>
> 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 <[email protected]>
> 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

2021-06-01 03:32:34

by Lai Jiangshan

Subject: [PATCH] KVM: X86: reset and read st->preempted in atomic way

From: Lai Jiangshan <[email protected]>

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 <[email protected]>

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

2021-06-08 00:38:54

by Sean Christopherson

Subject: Re: [PATCH] KVM: X86: reset and read st->preempted in atomic way

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 <[email protected]>
>
> 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 <[email protected]>

I saw this quirk too, but couldn't quite bring myself to care enought to test a
patch :-)

Reviewed-by: Sean Christopherson <[email protected]>