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Date: Thu, 10 Dec 2015 00:42:06 -0500
Message-ID: <CAMzpN2ibtL5BpSLf1D-s5MFiXQsD6TRnfNJ-eQ=HNqTmTQWoFg@mail.gmail.com>
Subject: Re: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path
From: Brian Gerst <brgerst@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On Wed, Dec 9, 2015 at 6:50 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> 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.

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