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Date: Mon, 7 Dec 2015 19:54:01 -0500
Message-ID: <CAMzpN2iNPA5TTSPht9WZqY-7u=Msd3UOZOJtJxt-nwG8+qEMeg@mail.gmail.com>
Subject: Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on
 the slow path
From: Brian Gerst <brgerst@gmail.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: "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 Mon, Dec 7, 2015 at 7:50 PM, Brian Gerst <brgerst@gmail.com> wrote:
> On Mon, Dec 7, 2015 at 4:51 PM, Andy Lutomirski <luto@kernel.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.

[Resend after gmail web interface fail]

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
    ret

This makes sure that the extra regs don't get saved a second time if
coming in from the slow path, but preserves the fast path if not
tracing.

--
Brian Gerst
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