* [PATCH] KVM: VMX: Check if bus lock vmexit was preempted
@ 2021-09-18 11:30 Hao Xiang
2021-09-22 10:02 ` Paolo Bonzini
0 siblings, 1 reply; 8+ messages in thread
From: Hao Xiang @ 2021-09-18 11:30 UTC (permalink / raw)
To: kvm
Cc: linux-kernel, chenyi.qiang, pbonzini, sean.j.christopherson,
shannon.zhao, Hao Xiang
exit_reason.bus_lock_detected is not only set when bus lock VM exit
was preempted, in fact, this bit is always set if bus locks are
detected no matter what the exit_reason.basic is.
So the bus_lock_vmexit handling in vmx_handle_exit should be duplicated
when exit_reason.basic is EXIT_REASON_BUS_LOCK(74). We can avoid it by
checking if bus lock vmexit was preempted in vmx_handle_exit.
Signed-off-by: Hao Xiang <hao.xiang@linux.alibaba.com>
---
arch/x86/kvm/vmx/vmx.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0c2c0d5..5ddf1df 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6054,7 +6054,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
* still need to exit to user space when bus lock detected to inform
* that there is a bus lock in guest.
*/
- if (to_vmx(vcpu)->exit_reason.bus_lock_detected) {
+ if (to_vmx(vcpu)->exit_reason.bus_lock_detected &&
+ to_vmx(vcpu)->exit_reason.basic != EXIT_REASON_BUS_LOCK) {
if (ret > 0)
vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
--
1.8.3.1
^ permalink raw reply related [flat|nested] 8+ messages in thread* Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted
2021-09-18 11:30 [PATCH] KVM: VMX: Check if bus lock vmexit was preempted Hao Xiang
@ 2021-09-22 10:02 ` Paolo Bonzini
2021-09-22 10:32 ` Xiaoyao Li
0 siblings, 1 reply; 8+ messages in thread
From: Paolo Bonzini @ 2021-09-22 10:02 UTC (permalink / raw)
To: Hao Xiang, kvm
Cc: linux-kernel, chenyi.qiang, sean.j.christopherson, shannon.zhao
On 18/09/21 13:30, Hao Xiang wrote:
> exit_reason.bus_lock_detected is not only set when bus lock VM exit
> was preempted, in fact, this bit is always set if bus locks are
> detected no matter what the exit_reason.basic is.
>
> So the bus_lock_vmexit handling in vmx_handle_exit should be duplicated
> when exit_reason.basic is EXIT_REASON_BUS_LOCK(74). We can avoid it by
> checking if bus lock vmexit was preempted in vmx_handle_exit.
I don't understand, does this mean that bus_lock_detected=1 if
basic=EXIT_REASON_BUS_LOCK? If so, can we instead replace the contents
of handle_bus_lock_vmexit with
/* Do nothing and let vmx_handle_exit exit to userspace. */
WARN_ON(!to_vmx(vcpu)->exit_reason.bus_lock_detected);
return 0;
?
That would be doable only if this is architectural behavior and not a
processor erratum, of course.
Thanks,
Paolo
> Signed-off-by: Hao Xiang <hao.xiang@linux.alibaba.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 0c2c0d5..5ddf1df 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6054,7 +6054,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
> * still need to exit to user space when bus lock detected to inform
> * that there is a bus lock in guest.
> */
> - if (to_vmx(vcpu)->exit_reason.bus_lock_detected) {
> + if (to_vmx(vcpu)->exit_reason.bus_lock_detected &&
> + to_vmx(vcpu)->exit_reason.basic != EXIT_REASON_BUS_LOCK) {
> if (ret > 0)
> vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
>
>
^ permalink raw reply [flat|nested] 8+ messages in thread* Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted
2021-09-22 10:02 ` Paolo Bonzini
@ 2021-09-22 10:32 ` Xiaoyao Li
2021-09-22 12:40 ` Paolo Bonzini
2021-09-22 14:58 ` Sean Christopherson
0 siblings, 2 replies; 8+ messages in thread
From: Xiaoyao Li @ 2021-09-22 10:32 UTC (permalink / raw)
To: Paolo Bonzini, Hao Xiang, kvm
Cc: linux-kernel, chenyi.qiang, shannon.zhao, Sean Christopherson
On 9/22/2021 6:02 PM, Paolo Bonzini wrote:
> On 18/09/21 13:30, Hao Xiang wrote:
>> exit_reason.bus_lock_detected is not only set when bus lock VM exit
>> was preempted, in fact, this bit is always set if bus locks are
>> detected no matter what the exit_reason.basic is.
>>
>> So the bus_lock_vmexit handling in vmx_handle_exit should be duplicated
>> when exit_reason.basic is EXIT_REASON_BUS_LOCK(74). We can avoid it by
>> checking if bus lock vmexit was preempted in vmx_handle_exit.
>
> I don't understand, does this mean that bus_lock_detected=1 if
> basic=EXIT_REASON_BUS_LOCK? If so, can we instead replace the contents
> of handle_bus_lock_vmexit with
>
> /* Do nothing and let vmx_handle_exit exit to userspace. */
> WARN_ON(!to_vmx(vcpu)->exit_reason.bus_lock_detected);
> return 0;
>
> ?
>
> That would be doable only if this is architectural behavior and not a
> processor erratum, of course.
EXIT_REASON.bus_lock_detected may or may not be set when exit reason ==
EXIT_REASON_BUS_LOCK. Intel will update ISE or SDM to state it.
Maybe we can do below in handle_bus_lock_vmexit handler:
if (!to_vmx(vcpu)->exit_reason.bus_lock_detected)
to_vmx(vcpu)->exit_reason.bus_lock_detected = 1;
But is manually changing the hardware reported value for software
purpose a good thing?
> Thanks,
>
> Paolo
>
>> Signed-off-by: Hao Xiang <hao.xiang@linux.alibaba.com>
>> ---
>> arch/x86/kvm/vmx/vmx.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 0c2c0d5..5ddf1df 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -6054,7 +6054,8 @@ static int vmx_handle_exit(struct kvm_vcpu
>> *vcpu, fastpath_t exit_fastpath)
>> * still need to exit to user space when bus lock detected to
>> inform
>> * that there is a bus lock in guest.
>> */
>> - if (to_vmx(vcpu)->exit_reason.bus_lock_detected) {
>> + if (to_vmx(vcpu)->exit_reason.bus_lock_detected &&
>> + to_vmx(vcpu)->exit_reason.basic != EXIT_REASON_BUS_LOCK) {
>> if (ret > 0)
>> vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
>>
>
^ permalink raw reply [flat|nested] 8+ messages in thread* Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted
2021-09-22 10:32 ` Xiaoyao Li
@ 2021-09-22 12:40 ` Paolo Bonzini
2021-09-23 0:59 ` Hao Xiang
2021-09-22 14:58 ` Sean Christopherson
1 sibling, 1 reply; 8+ messages in thread
From: Paolo Bonzini @ 2021-09-22 12:40 UTC (permalink / raw)
To: Xiaoyao Li, Hao Xiang, kvm
Cc: linux-kernel, chenyi.qiang, shannon.zhao, Sean Christopherson
On 22/09/21 12:32, Xiaoyao Li wrote:
>>
>
> EXIT_REASON.bus_lock_detected may or may not be set when exit reason ==
> EXIT_REASON_BUS_LOCK. Intel will update ISE or SDM to state it.
>
> Maybe we can do below in handle_bus_lock_vmexit handler:
>
> if (!to_vmx(vcpu)->exit_reason.bus_lock_detected)
> to_vmx(vcpu)->exit_reason.bus_lock_detected = 1;
>
> But is manually changing the hardware reported value for software
> purpose a good thing?
No. That said, Hao's patch is just making the code clearer; there's no
behavioral change since the "if" will just redo the same assignments as
handle_bus_lock_vmexit.
Paolo
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted
2021-09-22 12:40 ` Paolo Bonzini
@ 2021-09-23 0:59 ` Hao Xiang
2021-09-23 6:37 ` Paolo Bonzini
0 siblings, 1 reply; 8+ messages in thread
From: Hao Xiang @ 2021-09-23 0:59 UTC (permalink / raw)
To: Paolo Bonzini, Xiaoyao Li, kvm
Cc: linux-kernel, chenyi.qiang, shannon.zhao, Sean Christopherson, hao.xiang
On 2021/9/22 20:40, Paolo Bonzini wrote:
> On 22/09/21 12:32, Xiaoyao Li wrote:
>>>
>>
>> EXIT_REASON.bus_lock_detected may or may not be set when exit reason
>> == EXIT_REASON_BUS_LOCK. Intel will update ISE or SDM to state it.
>>
>> Maybe we can do below in handle_bus_lock_vmexit handler:
>>
>> if (!to_vmx(vcpu)->exit_reason.bus_lock_detected)
>> to_vmx(vcpu)->exit_reason.bus_lock_detected = 1;
>>
>> But is manually changing the hardware reported value for software
>> purpose a good thing?
>
> No. That said, Hao's patch is just making the code clearer; there's
> no behavioral change since the "if" will just redo the same
> assignments as handle_bus_lock_vmexit.
>
> Paolo
I agree Paolo. EXIT_REASON.bus_lock_detected may or may not be set when
exit_reason=EXIT_REASON_BUS_LOCK, It clould depend on hardware
implementaion. No matter when intel states it clearly, I think it is
better that we avoid repeated assignment by adding additional check
condition in vmx_handle_exit. Of course , it is also ok that
hand_bus_lock_vmexit do nothing , but the code is not clear, and the
code logic will be inconsistent with spec description.
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted
2021-09-23 0:59 ` Hao Xiang
@ 2021-09-23 6:37 ` Paolo Bonzini
0 siblings, 0 replies; 8+ messages in thread
From: Paolo Bonzini @ 2021-09-23 6:37 UTC (permalink / raw)
To: Hao Xiang, Xiaoyao Li, kvm
Cc: linux-kernel, chenyi.qiang, shannon.zhao, Sean Christopherson
On 23/09/21 02:59, Hao Xiang wrote:
> EXIT_REASON.bus_lock_detected may or may not be set when
> exit_reason=EXIT_REASON_BUS_LOCK, It clould depend on hardware
> implementaion. No matter when intel states it clearly, I think it is
> better that we avoid repeated assignment by adding additional check
> condition in vmx_handle_exit. Of course , it is also ok that
> hand_bus_lock_vmexit do nothing , but the code is not clear, and the
> code logic will be inconsistent with spec description.
For 5.16 we'll go with something like Sean's sketch, that sets the
bus_lock_detected bit on EXIT_REASON_BUS_LOCK.
Paolo
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted
2021-09-22 10:32 ` Xiaoyao Li
2021-09-22 12:40 ` Paolo Bonzini
@ 2021-09-22 14:58 ` Sean Christopherson
2021-09-23 7:15 ` Hao Xiang
1 sibling, 1 reply; 8+ messages in thread
From: Sean Christopherson @ 2021-09-22 14:58 UTC (permalink / raw)
To: Xiaoyao Li
Cc: Paolo Bonzini, Hao Xiang, kvm, linux-kernel, chenyi.qiang, shannon.zhao
On Wed, Sep 22, 2021, Xiaoyao Li wrote:
> On 9/22/2021 6:02 PM, Paolo Bonzini wrote:
> > On 18/09/21 13:30, Hao Xiang wrote:
> > > exit_reason.bus_lock_detected is not only set when bus lock VM exit
> > > was preempted, in fact, this bit is always set if bus locks are
> > > detected no matter what the exit_reason.basic is.
> > >
> > > So the bus_lock_vmexit handling in vmx_handle_exit should be duplicated
> > > when exit_reason.basic is EXIT_REASON_BUS_LOCK(74). We can avoid it by
> > > checking if bus lock vmexit was preempted in vmx_handle_exit.
> >
> > I don't understand, does this mean that bus_lock_detected=1 if
> > basic=EXIT_REASON_BUS_LOCK? If so, can we instead replace the contents
> > of handle_bus_lock_vmexit with
> >
> > /* Do nothing and let vmx_handle_exit exit to userspace. */
> > WARN_ON(!to_vmx(vcpu)->exit_reason.bus_lock_detected);
> > return 0;
> >
> > ?
> >
> > That would be doable only if this is architectural behavior and not a
> > processor erratum, of course.
>
> EXIT_REASON.bus_lock_detected may or may not be set when exit reason ==
> EXIT_REASON_BUS_LOCK. Intel will update ISE or SDM to state it.
>
> Maybe we can do below in handle_bus_lock_vmexit handler:
>
> if (!to_vmx(vcpu)->exit_reason.bus_lock_detected)
> to_vmx(vcpu)->exit_reason.bus_lock_detected = 1;
>
> But is manually changing the hardware reported value for software purpose a
> good thing?
In this case, I'd say yes. Hardware having non-deterministic behavior is the not
good thing, KVM would simply be correctly the not-technically-an-erratum erratum.
Set it unconditionally and then handle everything in common path. This has the
added advantage of having only one site that deals with KVM_RUN_X86_BUS_LOCK.
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 33f92febe3ce..aa9372452e49 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5561,9 +5561,9 @@ static int handle_encls(struct kvm_vcpu *vcpu)
static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
{
- vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
- vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
- return 0;
+ /* The dedicated flag may or may not be set by hardware. /facepalm. */
+ vcpu->exit_reason.bus_lock_detected = true;
+ return 1;
}
/*
@@ -6050,9 +6050,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
int ret = __vmx_handle_exit(vcpu, exit_fastpath);
/*
- * Even when current exit reason is handled by KVM internally, we
- * still need to exit to user space when bus lock detected to inform
- * that there is a bus lock in guest.
+ * Exit to user space when bus lock detected to inform that there is a
+ * bus lock in guest.
*/
if (to_vmx(vcpu)->exit_reason.bus_lock_detected) {
if (ret > 0)
^ permalink raw reply related [flat|nested] 8+ messages in thread* Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted
2021-09-22 14:58 ` Sean Christopherson
@ 2021-09-23 7:15 ` Hao Xiang
0 siblings, 0 replies; 8+ messages in thread
From: Hao Xiang @ 2021-09-23 7:15 UTC (permalink / raw)
To: Sean Christopherson, Xiaoyao Li
Cc: Paolo Bonzini, kvm, linux-kernel, chenyi.qiang, shannon.zhao
On 2021/9/22 22:58, Sean Christopherson wrote:
> On Wed, Sep 22, 2021, Xiaoyao Li wrote:
>> On 9/22/2021 6:02 PM, Paolo Bonzini wrote:
>>> On 18/09/21 13:30, Hao Xiang wrote:
>>>> exit_reason.bus_lock_detected is not only set when bus lock VM exit
>>>> was preempted, in fact, this bit is always set if bus locks are
>>>> detected no matter what the exit_reason.basic is.
>>>>
>>>> So the bus_lock_vmexit handling in vmx_handle_exit should be duplicated
>>>> when exit_reason.basic is EXIT_REASON_BUS_LOCK(74). We can avoid it by
>>>> checking if bus lock vmexit was preempted in vmx_handle_exit.
>>> I don't understand, does this mean that bus_lock_detected=1 if
>>> basic=EXIT_REASON_BUS_LOCK? If so, can we instead replace the contents
>>> of handle_bus_lock_vmexit with
>>>
>>> /* Do nothing and let vmx_handle_exit exit to userspace. */
>>> WARN_ON(!to_vmx(vcpu)->exit_reason.bus_lock_detected);
>>> return 0;
>>>
>>> ?
>>>
>>> That would be doable only if this is architectural behavior and not a
>>> processor erratum, of course.
>> EXIT_REASON.bus_lock_detected may or may not be set when exit reason ==
>> EXIT_REASON_BUS_LOCK. Intel will update ISE or SDM to state it.
>>
>> Maybe we can do below in handle_bus_lock_vmexit handler:
>>
>> if (!to_vmx(vcpu)->exit_reason.bus_lock_detected)
>> to_vmx(vcpu)->exit_reason.bus_lock_detected = 1;
>>
>> But is manually changing the hardware reported value for software purpose a
>> good thing?
> In this case, I'd say yes. Hardware having non-deterministic behavior is the not
> good thing, KVM would simply be correctly the not-technically-an-erratum erratum.
>
> Set it unconditionally and then handle everything in common path. This has the
> added advantage of having only one site that deals with KVM_RUN_X86_BUS_LOCK.
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 33f92febe3ce..aa9372452e49 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5561,9 +5561,9 @@ static int handle_encls(struct kvm_vcpu *vcpu)
>
> static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
> {
> - vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
> - vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
> - return 0;
> + /* The dedicated flag may or may not be set by hardware. /facepalm. */
> + vcpu->exit_reason.bus_lock_detected = true;
> + return 1;
> }
>
> /*
> @@ -6050,9 +6050,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
> int ret = __vmx_handle_exit(vcpu, exit_fastpath);
>
> /*
> - * Even when current exit reason is handled by KVM internally, we
> - * still need to exit to user space when bus lock detected to inform
> - * that there is a bus lock in guest.
> + * Exit to user space when bus lock detected to inform that there is a
> + * bus lock in guest.
> */
> if (to_vmx(vcpu)->exit_reason.bus_lock_detected) {
> if (ret > 0)
I agree with your modifications. And I will re-submit the patch. Thanks.
^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2021-09-23 7:15 UTC | newest]
Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-09-18 11:30 [PATCH] KVM: VMX: Check if bus lock vmexit was preempted Hao Xiang
2021-09-22 10:02 ` Paolo Bonzini
2021-09-22 10:32 ` Xiaoyao Li
2021-09-22 12:40 ` Paolo Bonzini
2021-09-23 0:59 ` Hao Xiang
2021-09-23 6:37 ` Paolo Bonzini
2021-09-22 14:58 ` Sean Christopherson
2021-09-23 7:15 ` Hao Xiang
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