linux-kernel - Re: [PATCH v5 09/11] KVM: X86: Add vendor callbacks for writing the TSC multiplier

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Date:   Tue, 1 Jun 2021 15:22:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ilias Stamatis <ilstam@...zon.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, mlevitsk@...hat.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        zamsden@...il.com, mtosatti@...hat.com, dwmw@...zon.co.uk
Subject: Re: [PATCH v5 09/11] KVM: X86: Add vendor callbacks for writing the
 TSC multiplier

On Fri, May 28, 2021, Ilias Stamatis wrote:
> @@ -10343,8 +10360,6 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>  	else
>  		vcpu->arch.mp_state = KVM_MP_STATE_UNINITIALIZED;
>  
> -	kvm_set_tsc_khz(vcpu, max_tsc_khz);
> -
>  	r = kvm_mmu_create(vcpu);
>  	if (r < 0)
>  		return r;
> @@ -10443,6 +10458,7 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
>  	if (mutex_lock_killable(&vcpu->mutex))
>  		return;
>  	vcpu_load(vcpu);
> +	kvm_set_tsc_khz(vcpu, max_tsc_khz);

It would be safer to move the call within kvm_arch_vcpu_create(), not to
postcreate().  postcreate() runs after the vCPU file descriptor is exposed to
userspace, e.g. KVM_GET_TSC_KHZ could theoretically read the wrong value.  I
don't know if there's anything more sinister that could be done, but I also
don't see any reason to find out :-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bc6160db94b1..08f6b8f82506 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10291,8 +10291,6 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
        else
                vcpu->arch.mp_state = KVM_MP_STATE_UNINITIALIZED;

-       kvm_set_tsc_khz(vcpu, max_tsc_khz);
-
        r = kvm_mmu_create(vcpu);
        if (r < 0)
                return r;
@@ -10360,6 +10358,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
        vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
        kvm_vcpu_mtrr_init(vcpu);
        vcpu_load(vcpu);
+       kvm_set_tsc_khz(vcpu, max_tsc_khz);
        kvm_vcpu_reset(vcpu, false);
        kvm_init_mmu(vcpu, false);
        vcpu_put(vcpu);



>  	kvm_synchronize_tsc(vcpu, 0);
>  	vcpu_put(vcpu);
>  
> -- 
> 2.17.1
>