linux-kernel - Re: [PATCH 1/2] KVM: X86: Move ignore

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Date:   Fri, 26 Jun 2020 14:07:32 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack

On Thu, Jun 25, 2020 at 09:25:40AM -0700, Sean Christopherson wrote:
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 5eb618dbf211..64322446e590 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1013,9 +1013,9 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>                 *ebx = entry->ebx;
>                 *ecx = entry->ecx;
>                 *edx = entry->edx;
> -               if (function == 7 && index == 0) {
> +               if (function == 7 && index == 0 && (*ebx | (F(RTM) | F(HLE))) {
>                         u64 data;
> -                       if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
> +                       if (!kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data) &&
>                             (data & TSX_CTRL_CPUID_CLEAR))
>                                 *ebx &= ~(F(RTM) | F(HLE));
>                 }
> 
> 
> On VMX, MSR_IA32_TSX_CTRL will be added to the so called shared MSR array
> regardless of whether or not it is being advertised to userspace (this is
> a bug in its own right).  Using the host_initiated variant means KVM will
> incorrectly bypass VMX's ARCH_CAP_TSX_CTRL_MSR check, i.e. incorrectly
> clear the bits if userspace is being weird and stuffed MSR_IA32_TSX_CTRL
> without advertising it to the guest.

Btw, would it be more staightforward to check "vcpu->arch.arch_capabilities &
ARCH_CAP_TSX_CTRL_MSR" rather than "*ebx | (F(RTM) | F(HLE))" even if we want
to have such a fix?

-- 
Peter Xu