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

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Feb 10, 2025 at 06:06:38PM +0000, Will Deacon wrote:
> 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.

Yep, that was the idea!

To avoid confusion: I've sent out v3 of this series *without* the
change, and I'll prepare that as a follow-up.

Mark.