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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH] selftests: kvm: fix sync_regs_test with newer gccs
In-Reply-To: <5b1b95e5-4836-ab55-fe4d-e9cc78a7a95e@redhat.com>
References: <20191008180808.14181-1-vkuznets@redhat.com> <20191008183634.GF14020@linux.intel.com> <b7d20806-4e88-91af-31c1-8cbb0a8a330b@redhat.com> <87d0f6yzd3.fsf@vitty.brq.redhat.com> <5b1b95e5-4836-ab55-fe4d-e9cc78a7a95e@redhat.com>
Date:   Wed, 09 Oct 2019 14:29:45 +0200
Message-ID: <874l0iyudi.fsf@vitty.brq.redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Paolo Bonzini <pbonzini@redhat.com> writes:

> 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.
>

Right, we're always resuming after so potential clobbering doesn't
matter. Thanks!

-- 
Vitaly