Re: [PATCH v2 04/12] x86/xen: drop USERGS_SYSRET64 paravirt call

[Borislav Petkov]

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

?

-- 
Regards/Gruss,
    Boris.

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