Re: [PATCH v2 1/8] KVM: arm64: Unconditionally save+flush host FPSIMD/SVE/SME state

[Will Deacon <will@kernel.org>]

On Thu, Feb 06, 2025 at 02:10:55PM +0000, Mark Rutland wrote:
> There are several problems with the way hyp code lazily saves the host's
> FPSIMD/SVE state, including:
> 
> * Host SVE being discarded unexpectedly due to inconsistent
>   configuration of TIF_SVE and CPACR_ELx.ZEN. This has been seen to
>   result in QEMU crashes where SVE is used by memmove(), as reported by
>   Eric Auger:
> 
>   https://issues.redhat.com/browse/RHEL-68997
> 
> * Host SVE state is discarded *after* modification by ptrace, which was an
>   unintentional ptrace ABI change introduced with lazy discarding of SVE state.
> 
> * The host FPMR value can be discarded when running a non-protected VM,
>   where FPMR support is not exposed to a VM, and that VM uses
>   FPSIMD/SVE. In these cases the hyp code does not save the host's FPMR
>   before unbinding the host's FPSIMD/SVE/SME state, leaving a stale
>   value in memory.

How hard would it be to write tests for these three scenarios? If we
had something to exercise the relevant paths then...

> ... and so this eager save+flush probably needs to be backported to ALL
> stable trees.

... this backporting might be a little easier to be sure about?

Will