linux-kernel - Re: [PATCH 11/15] KVM: VMX: Disable loading of TSX

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Date:   Wed, 5 May 2021 17:50:35 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...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, Xiaoyao Li <xiaoyao.li@...el.com>,
        Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH 11/15] KVM: VMX: Disable loading of TSX_CTRL MSR the more
 conventional way

On 05/05/21 17:36, Sean Christopherson wrote:
> On Wed, May 05, 2021, Paolo Bonzini wrote:
>> On 04/05/21 19:17, Sean Christopherson wrote:
>>> Tag TSX_CTRL as not needing to be loaded when RTM isn't supported in the
>>> host.  Crushing the write mask to '0' has the same effect, but requires
>>> more mental gymnastics to understand.
>>
>> This doesn't explain _why_ this is now possible.  What about:
>>
>> Now that user return MSRs is always present in the list, we don't have
> 
> User return MSRs aren't always present in the list; this series doesn't change
> that behavior at all.
> 
>> the problem that the TSX_CTRL MSR needs a slot vmx->guest_uret_msrs even
>> if RTM is not supported in the host (and therefore there is nothing to
>> enable).  Thus we can simply tag TSX_CTRL as not needing to be loaded
>> instead of crushing the write mask to '0'.
> 
> Unless I'm missing something, it would have been possible to give TSX_CTRL a
> slot but not load it even before this refactoring, we just missed that approach
> when handling the TSX_CTRL without HLE/RTM case.  Several other MSRs rely on
> this behavior, notably the SYSCALL MSRs, which are present in the list so that
> the guest can read/write the MSRs, but are loaded into hardware iff the guest
> has enabled SYSCALL.

You're right, it used to be done with vmx->nr_active_uret_msr.

Paolo

> All that said, I certainly have no objection to writing a longer changelog.