linux-kernel - Re: [PATCH] KVM: Fix steal time asm constraints in 32-bit mode

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Date:   Tue, 16 Nov 2021 10:46:29 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        kernel test robot <lkp@...el.com>, kvm <kvm@...r.kernel.org>
Cc:     kbuild-all@...ts.01.org, linux-kernel@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>, karahmed@...zon.com
Subject: Re: [PATCH] KVM: Fix steal time asm constraints in 32-bit mode

On 11/14/21 09:59, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> In 64-bit mode, x86 instruction encoding allows us to use the low 8 bits
> of any GPR as an 8-bit operand. In 32-bit mode, however, we can only use
> the [abcd] registers. For which, GCC has the "q" constraint instead of
> the less restrictive "r".
> 
> Fixes: 7e2175ebd695 ("KVM: x86: Fix recording of guest steal time / preempted status")
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   arch/x86/kvm/x86.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8f156905ae38..0a689bb62e9e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3307,7 +3307,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>   			     "xor %1, %1\n"
>   			     "2:\n"
>   			     _ASM_EXTABLE_UA(1b, 2b)
> -			     : "+r" (st_preempted),
> +			     : "+q" (st_preempted),
>   			       "+&r" (err)
>   			     : "m" (st->preempted));
>   		if (err)
> 

Queued with the addition of the "m" -> "+m" change, thanks.

Paolo