On Mon, 28 Jul 2025 at 17:10, Michael Tokarev <m...@tls.msk.ru> wrote:
>
> On 25.07.2025 20:55, Peter Maydell wrote:
> > A recent change to the kernel (Linux commit b376108e1f88
> > "arm64/fpsimd: signal: Clear TPIDR2 when delivering signals") updated
> > the signal-handler entry code to always clear TPIDR2_EL0.
> >
> > This is necessary for the userspace ZA lazy saving scheme to work
> > correctly when unwinding exceptions across a signal boundary.
> > (For the essay-length description of the incorrect behaviour and
> > why this is the correct fix, see the commit message for the
> > kernel commit.)
> >
> > Make QEMU also clear TPIDR2_EL0 on signal entry, applying the
> > equivalent bugfix to our implementation.
> >
> > Note that getting this unwinding to work correctly also requires
> > changes to the userspace code, e.g. as implemented in gcc in
> > https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=b5ffc8e75a8
> >
> > This change is technically an ABI change; from the kernel's
> > point of view SME was never enabled (it was hidden behind
> > CONFIG_BROKEN) before the change. From QEMU's point of view
> > our SME-related signal handling was broken anyway as we weren't
> > saving and restoring TPIDR2_EL0.
> >
> > Cc: qemu-sta...@nongnu.org
> > Fixes: 78011586b90d1 ("target/arm: Enable SME for user-only")
>
> Is it worth the efforts to apply this one to qemu 7.2.x branch?
Well, it's an easy backport since it's a one-liner, and
it's not a complicated change so it's pretty safe. There's
no point unless you're also backporting patch 2 of this series,
though.
In the 7.2.x version of target_setup_frame() the "clear
SVCR bits" code is a little different, but it's still there;
the change should go after this part:
if (env->svcr) {
env->svcr = 0;
arm_rebuild_hflags(env);
}
(there isn't actually an ordering requirement, so it's
just neater to put it in the same logical place)
-- PMM
