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

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Message-ID: <656c0af1-6c56-8a08-ff86-745409f6968c@redhat.com>
Date:   Wed, 20 Nov 2019 13:21:35 +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 19/11/19 22:06, Jim Mattson wrote:
>> +               switch (index) {
>> +               case MSR_IA32_TSX_CTRL:
>> +                       /* No need to pass TSX_CTRL_CPUID_CLEAR through.  */
>> +                       vmx->guest_msrs[j].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
>> +                       break;
> Why even bother with the special case here? Does this make the wrmsr faster?
> 

No, but it can avoid the wrmsr altogether if the guest uses the same
DISABLE_RTM setting but a different value for CPUID_CLEAR.

More important, while I am confident re-enabling TSX while in the kernel
and only restoring MSR_IA32_TSX_CTRL on return to userspace, I'm more
wary of changing CPUID bits while the kernel is running.  I will update
the comment.

Paolo