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

From: Andy Lutomirski <luto@amacapital.net>
To: Brian Gerst <brgerst@gmail.com>
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>,
	"Frédéric Weisbecker" <fweisbec@gmail.com>,
	"Denys Vlasenko" <dvlasenk@redhat.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path
Date: Wed, 9 Dec 2015 15:50:43 -0800	[thread overview]
Message-ID: <CALCETrX1aR57yULQ7mN+H5Um90=n3coXVKO5yz4JTCLpup7nGQ@mail.gmail.com> (raw)
In-Reply-To: <CALCETrUeY68J_OtmVTsUvyOyEvXCbJ9SuMZDbSvrp4BJXYVRHA@mail.gmail.com>

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