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Subject: Re: [PATCH] selftests: kvm: fix sync_regs_test with newer gccs
To:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jim Mattson <jmattson@google.com>
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 <20191008183634.GF14020@linux.intel.com>
 <b7d20806-4e88-91af-31c1-8cbb0a8a330b@redhat.com>
 <87d0f6yzd3.fsf@vitty.brq.redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Wed, 9 Oct 2019 13:11:24 +0200
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On 09/10/19 12:42, Vitaly Kuznetsov wrote:
> Paolo Bonzini <pbonzini@redhat.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