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+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"