linux-kernel - Re: [PATCH V4 04/50] x86/xen: Add xenpv_restore_regs_and_return_to

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Date:   Tue, 2 Nov 2021 17:19:46 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Borislav Petkov <bp@...en8.de>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Jan Beulich <jbeulich@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juergen Gross <jgross@...e.com>, Peter Anvin <hpa@...or.com>,
        xen-devel@...ts.xenproject.org, Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH V4 04/50] x86/xen: Add
 xenpv_restore_regs_and_return_to_usermode()

On 2021/11/2 16:58, Borislav Petkov wrote:

>>   	 */
>>   
>> -	ALTERNATIVE "", "jmp	swapgs_restore_regs_and_return_to_usermode", \
>> +	ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", \
> 
> Instead of sprinkling all those ALTERNATIVE calls everywhere,
> why don't you simply jump to the xenpv-one at the
> swapgs_restore_regs_and_return_to_usermode label itself and have a
> single ALTERNATIVE there?
> 

It will add a 5-byte NOP at the beginning of the native
swapgs_restore_regs_and_return_to_usermode.  I avoided adding unneeded
code in the native code even if it is NOPs and avoided melting xenpv-one into
the native one which will reduce the code readability.

I will follow your preference since a 5-byte NOP is so negligible in the slow
path with an iret instruction.

Or other option that adds macros to wrap the ALTERNATIVE.
RESTORE_REGS_AND_RETURN_TO_USERMODE and
COND_RESTORE_REGS_AND_RETURN_TO_USERMODE (test %eax before jmp in native case)