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

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Message-ID: <Y2vFIR+T+9SFY0xO@google.com>
Date:   Wed, 9 Nov 2022 15:20:01 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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 Wed, Nov 09, 2022, Paolo Bonzini wrote:
> In preparation for moving vmload/vmsave to __svm_vcpu_run,

__svm_vcpu_run()

> 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.

> No functional change intended.
> 
> Cc: stable@...r.kernel.org
> Fixes: a149180fbcf3 ("x86: Add magic AMD return-thunk")
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Reviewed-by: Sean Christopherson <seanjc@...gle.com>