linux-kernel - Re: [PATCH 1/2] x86/entry/64: Do not clear %rbx under Xen

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Date:   Sat, 21 Jul 2018 17:19:36 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     "M. Vefa Bicakci" <m.v.b@...box.com>, linux-kernel@...r.kernel.org
Cc:     Dominik Brodowski <linux@...inikbrodowski.net>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/entry/64: Do not clear %rbx under Xen

On 07/21/2018 03:49 PM, M. Vefa Bicakci wrote:
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index c7449f377a77..96e8ff34129e 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1129,7 +1129,7 @@ ENTRY(xen_failsafe_callback)
>  	addq	$0x30, %rsp
>  	UNWIND_HINT_IRET_REGS
>  	pushq	$-1 /* orig_ax = -1 => not a system call */
> -	PUSH_AND_CLEAR_REGS
> +	PUSH_AND_CLEAR_REGS clear_rbx=0


Do we need this at all? We are returning from the hypervisor here.

-boris

>  	ENCODE_FRAME_POINTER
>  	jmp	error_exit
>  END(xen_failsafe_callback)