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SubjectRe: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path
On Wed, Dec 9, 2015 at 1:15 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Dec 9, 2015 at 1:08 PM, Brian Gerst <brgerst@gmail.com> wrote:
>> Simplified version:
>> ENTRY(stub_ptregs_64)
>> cmpl $fast_path_return, (%rsp)
>
> Does that instruction actually work the way you want it to? (Does it
> link?) I think you might need to use leaq the way I did in my patch.
>
>> jne 1f
>> SAVE_EXTRA_REGS offset=8
>> call *%rax
>> RESTORE_EXTRA_REGS offset=8
>> ret
>> 1:
>> jmp *%rax
>> END(stub_ptregs_64)
>
> This'll work, I think, but I still think I prefer keeping as much
> complexity as possible in the slow path. I could be convinced
> otherwise, though -- this variant is reasonably clean.

On further reflection, there's at least one functional difference.
With my variant, modifying pt_regs from sys_foo/ptregs is safe. In
your variant, it's unsafe unless force_iret() is called. I don't know
whether we care.

--Andy


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