Messages in this thread | | | Date | Mon, 7 Dec 2015 19:54:01 -0500 | Subject | Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on the slow path | From | Brian Gerst <> |
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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.
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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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