linux-kernel - Re: [PATCH 06/14] KVM: x86: SVM: don't set VMLOAD/VMSAVE intercepts on vCPU reset

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Message-ID: <7289d070-152b-aece-1302-33e5a461657c@redhat.com>
Date:   Thu, 23 Sep 2021 18:40:51 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>, Bandan Das <bsd@...hat.com>,
        open list <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        Wei Huang <wei.huang2@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH 06/14] KVM: x86: SVM: don't set VMLOAD/VMSAVE intercepts
 on vCPU reset

On 14/09/21 17:48, Maxim Levitsky wrote:
> commit adc2a23734ac ("KVM: nSVM: improve SYSENTER emulation on AMD"),
> made init_vmcb set vmload/vmsave intercepts unconditionally,
> and relied on svm_vcpu_after_set_cpuid to clear them when possible.
> 
> However init_vmcb is also called when the vCPU is reset, and it is
> not followed by another call to svm_vcpu_after_set_cpuid because
> the CPUID is already set.
> 
> This mistake makes the VMSAVE/VMLOAD intercept to be set when
> it is not needed, and harms performance of the nested guest.
> 
> Fixes: adc2a23734ac ("KVM: nSVM: improve SYSENTER emulation on AMD")
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>   arch/x86/kvm/svm/svm.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 6645542df9bd..861ac9f74331 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1199,8 +1199,6 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>   	svm_set_intercept(svm, INTERCEPT_SHUTDOWN);
>   	svm_set_intercept(svm, INTERCEPT_VMRUN);
>   	svm_set_intercept(svm, INTERCEPT_VMMCALL);
> -	svm_set_intercept(svm, INTERCEPT_VMLOAD);
> -	svm_set_intercept(svm, INTERCEPT_VMSAVE);
>   	svm_set_intercept(svm, INTERCEPT_STGI);
>   	svm_set_intercept(svm, INTERCEPT_CLGI);
>   	svm_set_intercept(svm, INTERCEPT_SKINIT);
> @@ -1377,6 +1375,10 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
>   	svm->guest_state_loaded = false;
>   
>   	svm_switch_vmcb(svm, &svm->vmcb01);
> +
> +	svm_set_intercept(svm, INTERCEPT_VMLOAD);
> +	svm_set_intercept(svm, INTERCEPT_VMSAVE);
> +
>   	init_vmcb(vcpu);
>   
>   	svm_vcpu_init_msrpm(vcpu, svm->msrpm);
> 

This needs to be redone after the latest refactoring of svm_vcpu_reset. 
  I'll send a patch myself.

Paolo