linux-kernel - Re: [PATCH] selftests: kvm: fix sync_regs

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Message-ID: <5b1b95e5-4836-ab55-fe4d-e9cc78a7a95e@redhat.com>
Date:   Wed, 9 Oct 2019 13:11:24 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] selftests: kvm: fix sync_regs_test with newer gccs

On 09/10/19 12:42, Vitaly Kuznetsov wrote:
> Paolo Bonzini <pbonzini@...hat.com> writes:
>> There is no practical difference with Vitaly's patch.  The first
>> _vcpu_run has no pre-/post-conditions on the value of %rbx:
> 
> I think what Sean was suggesting is to prevent GCC from inserting
> anything (and thus clobbering RBX) between the call to guest_call() and
> the beginning of 'asm volatile' block by calling *inside* 'asm volatile'
> block instead.

Yes, but there is no way that clobbering RBX will break the test,
because RBX is not initialized until after the first _vcpu_run succeeds.

Paolo