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