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  1. 17 Mar, 2015 6 commits
  3. 10 Mar, 2015 3 commits
  5. 07 Mar, 2015 3 commits
    • Denys Vlasenko's avatar
      x86/asm: Optimize unnecessarily wide TEST instructions · 3e1aa7cb
      Denys Vlasenko authored 
      By the nature of the TEST operation, it is often possible to test
a narrower part of the operand:

    "testl $3,  mem"  ->  "testb $3, mem",
    "testq $3, %rcx"  ->  "testb $3, %cl"

This results in shorter instructions, because the TEST instruction
has no sign-entending byte-immediate forms unlike other ALU ops.

Note that this change does not create any LCP (Length-Changing Prefix)
stalls, which happen when adding a 0x66 prefix, which happens when
16-bit immediates are used, which changes such TEST instructions:

  [test_opcode] [modrm] [imm32]

to:

  [0x66] [test_opcode] [modrm] [imm16]

where [imm16] has a *different length* now: 2 bytes instead of 4.
This confuses the decoder and slows down execution.

REX prefixes were carefully designed to almost never hit this case:
adding REX prefix does not change instruction length except MOVABS
and MOV [addr],RAX instruction.

This patch does not add instructions which would use a 0x66 prefix,
code changes in assembly are:

    -48 f7 07 01 00 00 00 	testq  $0x1,(%rdi)
    +f6 07 01             	testb  $0x1,(%rdi)
    -48 f7 c1 01 00 00 00 	test   $0x1,%rcx
    +f6 c1 01             	test   $0x1,%cl
    -48 f7 c1 02 00 00 00 	test   $0x2,%rcx
    +f6 c1 02             	test   $0x2,%cl
    -41 f7 c2 01 00 00 00 	test   $0x1,%r10d
    +41 f6 c2 01          	test   $0x1,%r10b
    -48 f7 c1 04 00 00 00 	test   $0x4,%rcx
    +f6 c1 04             	test   $0x4,%cl
    -48 f7 c1 08 00 00 00 	test   $0x8,%rcx
    +f6 c1 08             	test   $0x8,%cl

Linus further notes:

   "There are no stalls from using 8-bit instruction forms.

    Now, changing from 64-bit or 32-bit 'test' instructions to 8-bit ones
    *could* cause problems if it ends up having forwarding issues, so that
    instead of just forwarding the result, you end up having to wait for
    it to be stable in the L1 cache (or possibly the register file). The
    forwarding from the store buffer is simplest and most reliable if the
    read is done at the exact same address and the exact same size as the
    write that gets forwarded.

    But that's true only if:

     (a) the write was very recent and is still in the write queue. I'm
         not sure that's the case here anyway.

     (b) on at least most Intel microarchitectures, you have to test a
         different byte than the lowest one (so forwarding a 64-bit write
         to a 8-bit read ends up working fine, as long as the 8-bit read
         is of the low 8 bits of the written data).

    A very similar issue *might* show up for registers too, not just
    memory writes, if you use 'testb' with a high-byte register (where
    instead of forwarding the value from the original producer it needs to
    go through the register file and then shifted). But it's mainly a
    problem for store buffers.

    But afaik, the way Denys changed the test instructions, neither of the
    above issues should be true.

    The real problem for store buffer forwarding tends to be "write 8
    bits, read 32 bits". That can be really surprisingly expensive,
    because the read ends up having to wait until the write has hit the
    cacheline, and we might talk tens of cycles of latency here. But
    "write 32 bits, read the low 8 bits" *should* be fast on pretty much
    all x86 chips, afaik."
Signed-off-by: default avatarDenys Vlasenko <dvlasenk@redhat.com>
Acked-by: default avatarAndy Lutomirski <luto@amacapital.net>
Acked-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1425675332-31576-1-git-send-email-dvlasenk@redhat.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
      3e1aa7cb
    • Andy Lutomirski's avatar
      x86/asm/entry: Replace this_cpu_sp0() with current_top_of_stack() and fix it on x86_32 · a7fcf28d
      Andy Lutomirski authored 
      I broke 32-bit kernels.  The implementation of sp0 was correct
as far as I can tell, but sp0 was much weirder on x86_32 than I
realized.  It has the following issues:

 - Init's sp0 is inconsistent with everything else's: non-init tasks
   are offset by 8 bytes.  (I have no idea why, and the comment is unhelpful.)

 - vm86 does crazy things to sp0.

Fix it up by replacing this_cpu_sp0() with
current_top_of_stack() and using a new percpu variable to track
the top of the stack on x86_32.
Signed-off-by: default avatarAndy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 75182b16 ("x86/asm/entry: Switch all C consumers of kernel_stack to this_cpu_sp0()")
Link: http://lkml.kernel.org/r/d09dbe270883433776e0cbee3c7079433349e96d.1425692936.git.luto@amacapital.netSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
      a7fcf28d
    • Andy Lutomirski's avatar
      x86/asm/entry: Delay loading sp0 slightly on task switch · b27559a4
      Andy Lutomirski authored 
      The change:

  75182b16 ("x86/asm/entry: Switch all C consumers of kernel_stack to this_cpu_sp0()")

had the unintended side effect of changing the return value of
current_thread_info() during part of the context switch process.
Change it back.

This has no effect as far as I can tell -- it's just for
consistency.
Signed-off-by: default avatarAndy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/9fcaa47dd8487db59eed7a3911b6ae409476763e.1425692936.git.luto@amacapital.netSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
      b27559a4
  7. 06 Mar, 2015 6 commits
  9. 04 Mar, 2015 22 commits
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