linux-kernel - Re: [PATCH v2 07/10] KVM: VMX: Drop explicit zeroing of MSR guest values at vCPU creation

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Message-ID: <87r1di7z0j.fsf@vitty.brq.redhat.com>
Date:   Tue, 21 Sep 2021 17:02:20 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH v2 07/10] KVM: VMX: Drop explicit zeroing of MSR guest
 values at vCPU creation

Sean Christopherson <seanjc@...gle.com> writes:

> Don't zero out user return and nested MSRs during vCPU creation, and
> instead rely on vcpu_vmx being zero-allocated.  Explicitly zeroing MSRs
> is not wrong, and is in fact necessary if KVM ever emulates vCPU RESET
> outside of vCPU creation, but zeroing only a subset of MSRs is confusing.
>
> Poking directly into KVM's backing is also undesirable in that it doesn't
> scale and is error prone.  Ideally KVM would have a common RESET path for
> all MSRs, e.g. by expanding kvm_set_msr(), which would obviate the need
> for this out-of-bad code (to support standalone RESET).

Just thinking out loud: we can probably enhance KVM_SET_MSRS making it
possible for userspace VMM to set the default (as not every setting need
to survive RESET). Conveniently enough, 'struct kvm_msr_entry' has 32
bits reserved and we can bite off one for 'default' flag.

>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d44d07d5a02f..8d14066db3ea 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6819,10 +6819,8 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
>  			goto free_vpid;
>  	}
>  
> -	for (i = 0; i < kvm_nr_uret_msrs; ++i) {
> -		vmx->guest_uret_msrs[i].data = 0;
> +	for (i = 0; i < kvm_nr_uret_msrs; ++i)
>  		vmx->guest_uret_msrs[i].mask = -1ull;
> -	}
>  	if (boot_cpu_has(X86_FEATURE_RTM)) {
>  		/*
>  		 * TSX_CTRL_CPUID_CLEAR is handled in the CPUID interception.
> @@ -6879,8 +6877,6 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
>  
>  	if (nested)
>  		memcpy(&vmx->nested.msrs, &vmcs_config.nested, sizeof(vmx->nested.msrs));
> -	else
> -		memset(&vmx->nested.msrs, 0, sizeof(vmx->nested.msrs));
>  
>  	vcpu_setup_sgx_lepubkeyhash(vcpu);

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

-- 
Vitaly