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

On Tue, Feb 04, 2025 at 06:00:24PM +0000, Marc Zyngier wrote:

On Tue, 04 Feb 2025 15:21:00 +0000,
Mark Rutland [off-list ref] wrote:

quoted

+static inline void fpsimd_lazy_switch_to_guest(struct kvm_vcpu *vcpu)
+{
+	u64 zcr_el1, zcr_el2;
+
+	if (!guest_owns_fp_regs())
+		return;
+
+	if (vcpu_has_sve(vcpu)) {
+		/* A guest hypervisor may restrict the effective max VL. */
+		if (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu))
+			zcr_el2 = __vcpu_sys_reg(vcpu, ZCR_EL2);
+		else
+			zcr_el2 = vcpu_sve_max_vq(vcpu) - 1;
+
+		sve_cond_update_zcr_vq(zcr_el2, SYS_ZCR_EL2);

Not a big deal, but I though I'd mention it here: Using ZCR_EL2 (or
any other register using the _EL2 suffix) is a source of expensive
traps with NV. We're much better off using the _EL1 accessor if we are
running VHE, as this will involve no trap at all.

nVHE will of course trap, but using nVHE with SVE under NV is not
something I'm prepared to give a damn about.

Ah, sorry. I had forgotten that wrinkle.

Given the compiler warnings reported by Mark Brown [1] and the kernel
test robot [2], I'll go spin a v2 with that cleaned up.

I'll use write_sysreg_el2() here, i.e.

	if (vcpu_has_sve(vcpu)) {
		/* A guest hypervisor may restrict the effective max VL. */
		if (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu))
			zcr_el2 = __vcpu_sys_reg(vcpu, ZCR_EL2);
		else
			zcr_el2 = vcpu_sve_max_vq(vcpu) - 1;

		write_sysreg_el2(scr_el2, SYS_ZCR);
	}

That'll use the preferred alias automatically, and it matches the style
used to write to ZCR_EL{1,12} immediately after.

Likewise for the other instances.

Mark.

[1] https://lore.kernel.org/linux-arm-kernel/b76803b7-c1b3-426b-a375-0c01b98142c9@sirena.org.uk/ (local)
[2] https://lore.kernel.org/oe-kbuild-all/202502061341.FvsCMKEH-lkp@intel.com/ (local)