[Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2198

[Alex Coplan]

Hi Philippe,

I'm aware of the prctl workaround.

It seems to me that this is clearly a regression in functionality. Prior
to the change, I could test any executable with any vector length
without having to modify the executable. Now I have to insert a prctl to
test with 1024 or 2048-bit SVE vectors?

Moreover, with this change, it's no longer possible to have the wider VL
inherited across exec() to another QEMU instance.

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https://bugs.launchpad.net/bugs/1906536

Title:
  Unable to set SVE VL to 1024 bits or above since 7b6a2198

Status in QEMU:
  New

Bug description:
  Prior to 7b6a2198e71794c851f39ac7a92d39692c786820, the QEMU option
  sve-max-vq could be used to set the vector length of the
  implementation. This is useful (among other reasons) for testing
  software compiled with a fixed SVE vector length. Since this commit,
  the vector length is capped at 512 bits.

  To reproduce the issue:

  $ cat rdvl.s
  .global _start
  _start:
    rdvl x0, #1
    asr x0, x0, #4
    mov x8, #93 // exit
    svc #0
  $ aarch64-linux-gnu-as -march=armv8.2-a+sve rdvl.s -o rdvl.o
  $ aarch64-linux-gnu-ld rdvl.o
  $ for vl in 1 2 4 8 16; do ../build-qemu/aarch64-linux-user/qemu-aarch64 -cpu 
max,sve-max-vq=$vl a.out; echo $?; done
  1
  2
  4
  4
  4

  For a QEMU built prior to the above revision, we get the output:
  1
  2
  4
  8
  16

  as expected. It seems that either the old behavior should be restored,
  or there should be an option to force a higher vector length?

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