Re: [PATCH v2 2/8] KVM: arm64: Remove host FPSIMD saving for non-protected KVM

[Will Deacon <will@kernel.org>]

On Mon, Feb 10, 2025 at 04:59:56PM +0000, Mark Rutland wrote:
> On Mon, Feb 10, 2025 at 04:12:43PM +0000, Will Deacon wrote:
> > On Thu, Feb 06, 2025 at 02:10:56PM +0000, Mark Rutland wrote:
> | static inline bool kvm_hyp_handle_fpsimd(struct kvm_vcpu *vcpu, u64 *exit_code)
> | {
> | 	...
> | 
> | 	/* Valid trap */
> | 
> | 	/*
> | 	 * Enable everything EL2 might need to save/restore state.
> | 	 * Maybe each of the bits should depend on system_has_xxx()
> | 	 */
> | 	cpacr_clear_set(0, CPACR_EL1_FPEN | CPACR_EL1_ZEN | CPACR_EL1_SMEN */
> | 	isb();
> | 
> | 	...
> | 
> | 	/* Write out the host state if it's in the registers */
> | 	if (is_protected_kvm_enabled() && host_owns_fp_regs())
> | 		kvm_hyp_save_fpsimd_host(vcpu);
> | 	
> | 	/* Restore guest state */
> | 
> | 	...
> | 
> | 	/*
> | 	 * Enable traps for the VCPU. The ERET will cause the traps to
> | 	 * take effect in the guest, so no ISB is necessary.
> | 	 */
> | 	cpacr_guest = CPACR_EL1_FPEN;
> | 	if (vcpu_has_sve(vcpu))
> | 		cpacr_guest |= CPACR_EL1_ZEN;
> | 	if (vcpu_has_sme(vcpu))			// whenever we add this
> | 		cpacr_guest |= CPACR_EL1_SMEN;
> | 	cpacr_clear_set(CPACR_EL1_FPEN | CPACR_EL1_ZEN | CPACR_EL1_SMEN,
> | 			cpacr_guest);
> | 
> | 	return true;
> | }
> 
> ... where we'd still have the CPACR write to re-enable traps, but it'd
> be unconditional, and wouldn't need an extra ISB.
> 
> If that makes sense to you, I can go spin that as a subsequent cleanup
> atop this series.

That looks very clean, yes please! Don't forget to drop the part from
kvm_hyp_save_fpsimd_host() too.

Will