Re: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path

[Brian Gerst]

On Wed, Dec 9, 2015 at 6:50 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Dec 9, 2015 at 1:15 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Wed, Dec 9, 2015 at 1:08 PM, Brian Gerst <brgerst@xxxxxxxxx> 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.

It should have been cmpq.  leaq isn't necessary, since immediates are
sign-extended to 64-bit.

>>>     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.

I can go either way at this point.  My main concern was getting rid of
the duplicate table.

--
Brian Gerst
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/