[PATCH 1/3] KVM: SVM: Get rid of handle_fastpath_set_msr_irqoff()

[PATCH 1/3] KVM: SVM: Get rid of handle_fastpath_set_msr_irqoff()

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

Analysis from Sean:

 | svm->next_rip is reset in svm_vcpu_run() only after calling 
 | svm_exit_handlers_fastpath(), which will cause SVM's 
 | skip_emulated_instruction() to write a stale RIP.
 
Let's get rid of handle_fastpath_set_msr_irqoff() in svm_exit_handlers_fastpath()
to have a quick fix.

Reported-by: Paul K. <kronenpj@kronenpj.dyndns.org>
Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Paul K. <kronenpj@kronenpj.dyndns.org>
Cc: <stable@vger.kernel.org> # v5.8-rc1+
Fixes: 404d5d7bff0d (KVM: X86: Introduce more exit_fastpath_completion enum values)
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/svm/svm.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 19e622a..c61bc3b 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3349,11 +3349,6 @@ static void svm_cancel_injection(struct kvm_vcpu *vcpu)
 
 static fastpath_t svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
 {
-	if (!is_guest_mode(vcpu) &&
-	    to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
-	    to_svm(vcpu)->vmcb->control.exit_info_1)
-		return handle_fastpath_set_msr_irqoff(vcpu);
-
 	return EXIT_FASTPATH_NONE;
 }
 
-- 
2.7.4

Re: [PATCH 1/3] KVM: SVM: Get rid of handle_fastpath_set_msr_irqoff()

[Vitaly Kuznetsov]

Wanpeng Li <kernellwp@gmail.com> writes:

> From: Wanpeng Li <wanpengli@tencent.com>
>
> Analysis from Sean:
>
>  | svm->next_rip is reset in svm_vcpu_run() only after calling 
>  | svm_exit_handlers_fastpath(), which will cause SVM's 
>  | skip_emulated_instruction() to write a stale RIP.
>  

This should only happen when svm->vmcb->control.next_rip is not set by
hardware as skip_emulated_instruction() itself sets 'svm->next_rip'
otherwise, right?

> Let's get rid of handle_fastpath_set_msr_irqoff() in svm_exit_handlers_fastpath()
> to have a quick fix.

which in the light of the whole seeries seems to be appropriate, so:

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

>
> Reported-by: Paul K. <kronenpj@kronenpj.dyndns.org>
> Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Cc: Paul K. <kronenpj@kronenpj.dyndns.org>
> Cc: <stable@vger.kernel.org> # v5.8-rc1+
> Fixes: 404d5d7bff0d (KVM: X86: Introduce more exit_fastpath_completion enum values)
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/svm/svm.c | 5 -----
>  1 file changed, 5 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 19e622a..c61bc3b 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3349,11 +3349,6 @@ static void svm_cancel_injection(struct kvm_vcpu *vcpu)
>  
>  static fastpath_t svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
>  {
> -	if (!is_guest_mode(vcpu) &&
> -	    to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
> -	    to_svm(vcpu)->vmcb->control.exit_info_1)
> -		return handle_fastpath_set_msr_irqoff(vcpu);
> -
>  	return EXIT_FASTPATH_NONE;
>  }

-- 
Vitaly

Re: [PATCH 1/3] KVM: SVM: Get rid of handle_fastpath_set_msr_irqoff()

[Wanpeng Li]

On Wed, 9 Sep 2020 at 16:23, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Wanpeng Li <kernellwp@gmail.com> writes:
>
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > Analysis from Sean:
> >
> >  | svm->next_rip is reset in svm_vcpu_run() only after calling
> >  | svm_exit_handlers_fastpath(), which will cause SVM's
> >  | skip_emulated_instruction() to write a stale RIP.
> >
>
> This should only happen when svm->vmcb->control.next_rip is not set by
> hardware as skip_emulated_instruction() itself sets 'svm->next_rip'
> otherwise, right?

The bug is reported here
https://bugzilla.kernel.org/show_bug.cgi?id=209155 , the old machine
which the reporter uses doesn't have NRIP save on #VMEXIT support. :)

    Wanpeng