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

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Date:	Wed, 9 Dec 2015 21:54:23 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path

On Wed, Dec 9, 2015 at 9:42 PM, Brian Gerst <brgerst@...il.com> wrote:
> On Wed, Dec 9, 2015 at 6:50 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Wed, Dec 9, 2015 at 1:15 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Wed, Dec 9, 2015 at 1:08 PM, Brian Gerst <brgerst@...il.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.
>
> It should have been cmpq.  leaq isn't necessary, since immediates are
> sign-extended to 64-bit.

Right, I always forget that they're sign-extended and not zero-extended.

I folded that bit in to my queue.

>
>>>>     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.

Agreed.  I'll sleep on it, and maybe someone else has some reason to
prefer one approach over the other.

--Andy



-- 
Andy Lutomirski
AMA Capital Management, LLC
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