id = 2304
title = "Disabling SVE via `-cpu max,sve=off` leaves SVE2 advertised by `getauxval`"
state = "closed"
created_at = "2024-04-23T12:16:36.267Z"
closed_at = "2024-08-01T08:31:57.918Z"
labels = ["Closed::Fixed", "linux-user", "target: arm"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2304"
host-os = "Ubuntu 22.04"
host-arch = "x86_64"
qemu-version = "qemu-aarch64 version 8.2.94"
guest-os = "n/a"
guest-arch = "n/a"
description = """The documentation on https://qemu-project.gitlab.io/qemu/system/arm/cpu-features.html suggests that it should be possible to disable SVE support by passing `-cpu max,sve=off` on the command line, however this appears to only disable the SVE support advertised in the return value from `getauxval(AT_HWCAP)`. In particular it leaves SVE2 reported as enabled. This leaves the feature set advertised by `getauxval` in an inconsistent state since SVE is mandatory if SVE2 is available.

This may also affect other feature dependencies for example FEAT_SVE_BITPerm also requiring SVE2 to be available, I've not checked exhaustively.

For example, given the following code:

    #include <sys/auxv.h>
    #include <stdio.h>

    int main() {
      unsigned long hwcap = getauxval(AT_HWCAP);
      unsigned long hwcap2 = getauxval(AT_HWCAP2);

      if (hwcap & HWCAP_SVE) {
        printf("have sve!\\n");
      } else {
        printf("don't have sve!\\n");
      }
      if (hwcap2 & HWCAP2_SVE2) {
        printf("have sve2!\\n");
      } else {
        printf("don't have sve2!\\n");
      }
    }

We can observe the following:

    $ aarch64-linux-gnu-gcc test.c -static
    $ ../qemu-aarch64 -cpu max ./a.out
    have sve!
    have sve2!
    $ ../qemu-aarch64 -cpu max,sve=off ./a.out
    don't have sve!
    have sve2!

I don't believe that there is a `-cpu ...,sve2=off` option, so I would expect that disabling SVE also prevents SVE2 from being advertised as available."""
reproduce = "n/a"
additional = "n/a"