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    SubjectRe: [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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