Re: [PATCH v3 8/8] KVM: arm64: Eagerly switch ZCR_EL{1,2}

[Will Deacon <will@kernel.org>]

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@arm.com>
> Reviewed-by: Mark Brown <broonie@kernel.org>
> Tested-by: Mark Brown <broonie@kernel.org>
> Cc: stable@vger.kernel.org
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Fuad Tabba <tabba@google.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Oliver Upton <oliver.upton@linux.dev>
> Cc: Will Deacon <will@kernel.org>
> ---
>  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@kernel.org>

Thanks for the quick re-spin!

Will