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

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Message-ID: <CAMzpN2ibtL5BpSLf1D-s5MFiXQsD6TRnfNJ-eQ=HNqTmTQWoFg@mail.gmail.com>
Date:	Thu, 10 Dec 2015 00:42:06 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
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 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.

>>>     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
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