linux-kernel - Re: [PATCH v2 04/12] x86/xen: drop USERGS

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Date:   Wed, 2 Dec 2020 15:48:21 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, peterz@...radead.org,
        luto@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Deep Shah <sdeep@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH v2 04/12] x86/xen: drop USERGS_SYSRET64 paravirt call

On 02.12.20 13:32, Borislav Petkov wrote:
> On Fri, Nov 20, 2020 at 12:46:22PM +0100, Juergen Gross wrote:
>> @@ -123,12 +115,15 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
>>   	 * Try to use SYSRET instead of IRET if we're returning to
>>   	 * a completely clean 64-bit userspace context.  If we're not,
>>   	 * go to the slow exit path.
>> +	 * In the Xen PV case we must use iret anyway.
>>   	 */
>> -	movq	RCX(%rsp), %rcx
>> -	movq	RIP(%rsp), %r11
>>   
>> -	cmpq	%rcx, %r11	/* SYSRET requires RCX == RIP */
>> -	jne	swapgs_restore_regs_and_return_to_usermode
>> +	ALTERNATIVE __stringify( \
>> +		movq	RCX(%rsp), %rcx; \
>> +		movq	RIP(%rsp), %r11; \
>> +		cmpq	%rcx, %r11;	/* SYSRET requires RCX == RIP */ \
>> +		jne	swapgs_restore_regs_and_return_to_usermode), \
>> +	"jmp	swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
> 
> Why such a big ALTERNATIVE when you can simply do:
> 
>          /*
>           * Try to use SYSRET instead of IRET if we're returning to
>           * a completely clean 64-bit userspace context.  If we're not,
>           * go to the slow exit path.
>           * In the Xen PV case we must use iret anyway.
>           */
>          ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
> 
>          movq    RCX(%rsp), %rcx;
>          movq    RIP(%rsp), %r11;
>          cmpq    %rcx, %r11;     /* SYSRET requires RCX == RIP */ \
>          jne     swapgs_restore_regs_and_return_to_usermode
> 
> ?
> 

I wanted to avoid the additional NOPs for the bare metal case.

If you don't mind them I can do as you are suggesting.


Juergen

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