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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