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

    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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