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Message-ID: <d4ae23dd-377e-8316-909b-d5bdeacc0904@linux.alibaba.com>
Date:   Tue, 2 Nov 2021 17:19:46 +0800
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Subject: Re: [PATCH V4 04/50] x86/xen: Add
 xenpv_restore_regs_and_return_to_usermode()
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To:     Borislav Petkov <bp@alien8.de>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Jan Beulich <jbeulich@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Juergen Gross <jgross@suse.com>, Peter Anvin <hpa@zytor.com>,
        xen-devel@lists.xenproject.org, Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>
References: <20211026141420.17138-1-jiangshanlai@gmail.com>
 <20211026141420.17138-5-jiangshanlai@gmail.com> <YYD9ohN2Zcy4EdMb@zn.tnic>
From:   Lai Jiangshan <laijs@linux.alibaba.com>
In-Reply-To: <YYD9ohN2Zcy4EdMb@zn.tnic>
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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)