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

[Alex Coplan <1906536@bugs.launchpad.net>]

Public bug reported:

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?

** Affects: qemu
     Importance: Undecided
         Status: New

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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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