On Fri, 4 Jul 2025 at 14:17, Philippe Mathieu-Daudé <phi...@linaro.org> wrote:
>
> On 4/7/25 15:03, Peter Maydell wrote:
> > On Mon, 30 Jun 2025 at 14:09, Philippe Mathieu-Daudé <phi...@linaro.org>
> > wrote:
> >>
> >> Only allow disabling NEON when using TCG.
> >>
> >> This avoids confusing user experience:
> >>
> >> $ qemu-system-aarch64 -M virt -accel hvf \
> >> -cpu host,neon=off,vfp=off,vfp-d32=off
> >> qemu-system-aarch64: AArch64 CPUs must have both VFP and Neon or neither
> >>
> >> $ qemu-system-aarch64 -M virt -accel hvf \
> >> -cpu host,neon=off,vfp=off,vfp-d32=off
> >> qemu-system-aarch64: ARM CPUs must have both VFP-D32 and Neon or neither
> >>
> >> $ qemu-system-aarch64 -M virt -accel hvf \
> >> -cpu host,neon=off,vfp=off,vfp-d32=off
> >> qemu-system-aarch64: can't apply global host-arm-cpu.vfp-d32=off:
> >> Property 'host-arm-cpu.vfp-d32' not found
> >
> > I don't know about users, but I'm definitely confused.
> > Aren't these three all the same command line ? What's
> > the problem it's trying to show ?
>
> I should have only shared this confusing example:
>
> $ qemu-system-aarch64 -M virt -accel hvf -cpu host,vfp-d32=off
> qemu-system-aarch64: can't apply global host-arm-cpu.vfp-d32=off:
> Property 'host-arm-cpu.vfp-d32' not found
That seems correct, though ? We say
* Allow user to turn off VFP and Neon support, but only for TCG --
* KVM does not currently allow us to lie to the guest about its
* ID/feature registers, so the guest always sees what the host has.
and hvf is currently the same. The bug the patch is fixing
looks like it is that -accel hvf lets us say '-cpu host,neon=off'
but it won't actually do what the user might be intending. So
we should avoid creating the property, as we already do for KVM.
(The other odd thing here is that hvf CPUs don't support
AArch32, but ARM_FEATURE_NEON is about AArch32. But that's
a separate thing and we'd need to cross-check that nothing
aarch64 is incorrectly looking at that feature flag before
we suppressed it for CPUs without any AArch32 support.)
-- PMM
