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

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Date:	Wed, 9 Dec 2015 15:50:43 -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 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.
>
>>     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
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