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SubjectRe: [PATCH 13/16] x86/entry/64: Create a percpu SYSCALL entry trampoline
On Mon, Nov 20, 2017 at 6:34 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Mon, Nov 20, 2017 at 09:07:44AM -0800, Andy Lutomirski wrote:
>> + /* Save RDI, since we need a scratch register. */
>> + pushq %rdi
>> +
>> + /*
>> + * x86 lacks a near absolute jump, and we can't jump to the real
>> + * entry text with a relative jump, so we use a double trampoline.
>> + */
>> + movq $entry_SYSCALL_64_stage2, %rdi
>> + jmp *%rdi
>> +END(entry_SYSCALL_64_trampoline)
>> +
>> + .popsection
>> +
>> +ENTRY(entry_SYSCALL_64_stage2)
>> + /*
>> + * Rather than polluting the normal SYSCALL path with stack switching
>> + * nonsense, fix up our register state to match its expectations.
>> + */
>> + UNWIND_HINT_EMPTY
>> + popq %rdi
>> + jmp entry_SYSCALL_64_after_hwframe
>> +END(entry_SYSCALL_64_stage2)
>
> Is there a reason why you couldn't just do the following?
>
> pushq $entry_SYSCALL_64_after_hwframe
> ret
>
> Then you wouldn't need the 2nd trampoline and the %rdi clobber.
>

Good suggestion. Thanks!

> --
> Josh

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