linux-kernel - Re: [PATCH 4/5] KVM: vmx: implement MSR_IA32_TSX

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Date:   Thu, 5 Dec 2019 11:16:16 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 4/5] KVM: vmx: implement MSR_IA32_TSX_CTRL disable RTM
 functionality

On 05/12/19 00:49, Jim Mattson wrote:
>>         if (!boot_cpu_has(X86_FEATURE_RTM))
>> -               data &= ~ARCH_CAP_TAA_NO;
>> +               data &= ~(ARCH_CAP_TAA_NO | ARCH_CAP_TSX_CTRL_MSR);
>>         else if (!boot_cpu_has_bug(X86_BUG_TAA))
>>                 data |= ARCH_CAP_TAA_NO;
>> -       else if (data & ARCH_CAP_TSX_CTRL_MSR)
>> -               data &= ~ARCH_CAP_MDS_NO;
>>
>> -       /* KVM does not emulate MSR_IA32_TSX_CTRL.  */
>> -       data &= ~ARCH_CAP_TSX_CTRL_MSR;
> Shouldn't kvm be masking off any bits that it doesn't know about here?
> Who knows what future features we may claim to support?

Good question, in the past the ARCH_CAPABILITIES were just "we don't
have this bug" so it made sense to pass everything through.  Now we have
TSX_CTRL that is of a different kind and arguably should have been a
CPUID bit, so we should indeed mask unknown capabilties.

Paolo