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



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.

https://people.kernel.org/tglx/notes-about-netiquette



 


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