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

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Message-ID: <20191009162230.GA31986@linux.intel.com>
Date:   Wed, 9 Oct 2019 09:22:31 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, 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 Wed, Oct 09, 2019 at 01:11:24PM +0200, Paolo Bonzini wrote:
> 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.

Ah, nice, wasn't aware of that.