linux-kernel - Re: [PATCH 03/11] KVM: SVM: adjust register allocation for __svm_vcpu

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Message-ID: <cf24a179-3c19-f4b2-d62d-3c812f46c4dc@redhat.com>
Date:   Wed, 9 Nov 2022 17:04:46 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        thomas.lendacky@....com, jmattson@...gle.com,
        stable@...r.kernel.org
Subject: Re: [PATCH 03/11] KVM: SVM: adjust register allocation for
 __svm_vcpu_run

On 11/9/22 16:20, Sean Christopherson wrote:
>> keep the pointer to the struct vcpu_svm in %rdi.  This way
>> it is possible to load svm->vmcb01.pa in %rax without
>> clobbering the pointer to svm itself.
>
> If you feel like doing fixup before pushing, add a note to call out that avoiding
> RAX will also be "necessary" to play nice with loading/storing MSR_SPEC_CTRL?
> When fiddling with this code, I found the RDMSR/WRMSR clobbers to be far more
> problematic than the VMCB pointers.

I tried to avoid the "in the future" phrasing which some people dislike. 
  I'll rephrase anyway.  Even without taking 
RDMSR/WRMSR/VMLOAD/VMRUN/VMSAVE into account, avoiding the potential 
clash with 32-bit register arguments is a decent reason for the patch 
anyway.

Paolo