On Mon, Feb 10, 2025 at 07:52:26PM +0000, Mark Rutland wrote:
> In non-protected KVM modes, while the guest FPSIMD/SVE/SME state is live on the
> CPU, the host's active SVE VL may differ from the guest's maximum SVE VL:
>
> * For VHE hosts, when a VM uses NV, ZCR_EL2 contains a value constrained
> by the guest hypervisor, which may be less than or equal to that
> guest's maximum VL.
>
> Note: in this case the value of ZCR_EL1 is immaterial due to E2H.
>
> * For nVHE/hVHE hosts, ZCR_EL1 contains a value written by the guest,
> which may be less than or greater than the guest's maximum VL.
>
> Note: in this case hyp code traps host SVE usage and lazily restores
> ZCR_EL2 to the host's maximum VL, which may be greater than the
> guest's maximum VL.
>
> This can be the case between exiting a guest and kvm_arch_vcpu_put_fp().
> If a softirq is taken during this period and the softirq handler tries
> to use kernel-mode NEON, then the kernel will fail to save the guest's
> FPSIMD/SVE state, and will pend a SIGKILL for the current thread.
>
> This happens because kvm_arch_vcpu_ctxsync_fp() binds the guest's live
> FPSIMD/SVE state with the guest's maximum SVE VL, and
> fpsimd_save_user_state() verifies that the live SVE VL is as expected
> before attempting to save the register state:
>
> | if (WARN_ON(sve_get_vl() != vl)) {
> | force_signal_inject(SIGKILL, SI_KERNEL, 0, 0);
> | return;
> | }
>
> Fix this and make this a bit easier to reason about by always eagerly
> switching ZCR_EL{1,2} at hyp during guest<->host transitions. With this
> happening, there's no need to trap host SVE usage, and the nVHE/nVHE
> __deactivate_cptr_traps() logic can be simplified to enable host access
> to all present FPSIMD/SVE/SME features.
>
> In protected nVHE/hVHE modes, the host's state is always saved/restored
> by hyp, and the guest's state is saved prior to exit to the host, so
> from the host's PoV the guest never has live FPSIMD/SVE/SME state, and
> the host's ZCR_EL1 is never clobbered by hyp.
>
> Fixes: 8c8010d69c132273 ("KVM: arm64: Save/restore SVE state for nVHE")
> Fixes: 2e3cf82063a00ea0 ("KVM: arm64: nv: Ensure correct VL is loaded before saving SVE state")
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Reviewed-by: Mark Brown <broonie@xxxxxxxxxx>
> Tested-by: Mark Brown <broonie@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Fuad Tabba <tabba@xxxxxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> ---
> arch/arm64/kvm/fpsimd.c | 30 -------------
> arch/arm64/kvm/hyp/entry.S | 5 +++
> arch/arm64/kvm/hyp/include/hyp/switch.h | 59 +++++++++++++++++++++++++
> arch/arm64/kvm/hyp/nvhe/hyp-main.c | 13 +++---
> arch/arm64/kvm/hyp/nvhe/switch.c | 6 +--
> arch/arm64/kvm/hyp/vhe/switch.c | 4 ++
> 6 files changed, 76 insertions(+), 41 deletions(-)
Acked-by: Will Deacon <will@xxxxxxxxxx>
Thanks for the quick re-spin!
Will
