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From: "goldstein.w.n at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug string/28354] New: For x86_64 string/memory functions use of
 EVEX registers sets HI16_ZMM_state adding context switch overhead
Date: Mon, 20 Sep 2021 02:23:27 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D28354

            Bug ID: 28354
           Summary: For x86_64 string/memory functions use of EVEX
                    registers sets HI16_ZMM_state adding context switch
                    overhead
           Product: glibc
           Version: 2.34
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: string
          Assignee: unassigned at sourceware dot org
          Reporter: goldstein.w.n at gmail dot com
  Target Milestone: ---

Use of ymm16-ymm31 in the exex string/memory functions in
sysdeps/x86_64/multtiarch sets HI16_ZMM_state to true.
See:
https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-=
32-architectures-software-developer-vol-1-manual.pdf#page=3D321

This defeats the init optimization of various xsave* context switching
instructions. Overall it adds at the very least 1024 bytes to context switc=
hes.

Simple reproduction:

```
        .global _start
        .text
_start:
    vpxorq  %ymm16, %ymm16, %ymm16
    vzeroupper

loop:
    jmp loop


        movl    $60, %eax
        xorl    %edi, %edi
        syscall
```

Then check:

cat /proc/${pid}/arch_status


Which will show the state being continuously updated.

State is updated during context switch here:
https://elixir.bootlin.com/linux/v5.15-rc1/source/arch/x86/kernel/fpu/core.=
c#L108)

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