linux-kernel - Re: [RFC][PATCH 15/22] x86,vmx: Remove .fixup usage

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Date:   Thu, 4 Nov 2021 19:50:38 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        mark.rutland@....com, dvyukov@...gle.com, seanjc@...gle.com,
        mbenes@...e.cz
Subject: Re: [RFC][PATCH 15/22] x86,vmx: Remove .fixup usage

On 11/4/21 17:47, Peter Zijlstra wrote:
> In the vmread exceptin path, use the, thus far, unused output register
> to push the @fault argument onto the stack. This, in turn, enables the
> exception handler to not do pushes and only modify that register when
> an exception does occur.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>   arch/x86/kvm/vmx/vmx_ops.h |   14 ++++++--------
>   1 file changed, 6 insertions(+), 8 deletions(-)
> 
> --- a/arch/x86/kvm/vmx/vmx_ops.h
> +++ b/arch/x86/kvm/vmx/vmx_ops.h
> @@ -80,9 +80,11 @@ static __always_inline unsigned long __v
>   		      * @field, and bounce through the trampoline to preserve
>   		      * volatile registers.
>   		      */
> -		     "push $0\n\t"
> +		     "xorl %k1, %k1\n\t"
> +		     "2:\n\t"
> +		     "push %1\n\t"
>   		     "push %2\n\t"
> -		     "2:call vmread_error_trampoline\n\t"
> +		     "call vmread_error_trampoline\n\t"
>   
>   		     /*
>   		      * Unwind the stack.  Note, the trampoline zeros out the
> @@ -93,12 +95,8 @@ static __always_inline unsigned long __v
>   		     "3:\n\t"
>   
>   		     /* VMREAD faulted.  As above, except push '1' for @fault. */
> -		     ".pushsection .fixup, \"ax\"\n\t"
> -		     "4: push $1\n\t"
> -		     "push %2\n\t"
> -		     "jmp 2b\n\t"
> -		     ".popsection\n\t"
> -		     _ASM_EXTABLE(1b, 4b)
> +		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_ONE_REG, %1)
> +
>   		     : ASM_CALL_CONSTRAINT, "=r"(value) : "r"(field) : "cc");
>   	return value;
>   }
> 
> 

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>