linux-kernel - Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on the slow path

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Message-ID: <CAMzpN2hGTMSHb7LEYC_w6grxCtGfuLmXoKhgH9vQ8hVHqmH2Fg@mail.gmail.com>
Date:	Mon, 7 Dec 2015 19:50:55 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"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 07/12] x86/entry/64: Always run ptregs-using syscalls on
 the slow path

On Mon, Dec 7, 2015 at 4:51 PM, Andy Lutomirski <luto@...nel.org> wrote:
> 64-bit syscalls currently have an optimization in which they are
> called with partial pt_regs.  A small handful require full pt_regs.
>
> In the 32-bit and compat cases, I cleaned this up by forcing full
> pt_regs for all syscalls.  The performance hit doesn't really matter.
>
> I want to clean up the 64-bit case as well, but I don't want to hurt
> fast path performance.  To do that, I want to force the syscalls
> that use pt_regs onto the slow path.  This will enable us to make
> slow path syscalls be real ABI-compliant C functions.
>
> Use the new syscall entry qualification machinery for this.
> stub_clone is now stub_clone/ptregs.
>
> The next patch will eliminate the stubs, and we'll just have
> sys_clone/ptregs.

I've got an idea on how to do this without the duplicate syscall table.

ptregs_foo:
    leaq sys_foo(%rip), %rax
    jmp stub_ptregs_64

stub_ptregs_64:
    testl $TS_EXTRAREGS, <current->ti_status>
    jnz 1f
    SAVE_EXTRA_REGS
    call *%rax
    RESTORE_EXTRA_REGS
    ret
1:
    call *%rax


--
Brian Gerst
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