Commit eef4ce63 authored by Fuad Tabba's avatar Fuad Tabba Committed by Marc Zyngier
Browse files

KVM: arm64: Clarify rationale for ZCR_EL1 value restored on guest exit 



Expand comment clarifying why the host value representing SVE
vector length being restored for ZCR_EL1 on guest exit isn't the
same as it was on guest entry.

Signed-off-by: default avatarFuad Tabba <tabba@google.com>
Reviewed-by: default avatarMark Brown <broonie@kernel.org>
Acked-by: default avatarOliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240423150538.2103045-21-tabba@google.com


Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
parent b6ed4fa9

arch/arm64/kvm/fpsimd.c

+23 −1
Original line number Diff line number Diff line
@@ -175,12 +175,34 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu) 
		if (vcpu_has_sve(vcpu)) {

			__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR);



			/* Restore the VL that was saved when bound to the CPU */

			/*

			 * Restore the VL that was saved when bound to the CPU,

			 * which is the maximum VL for the guest. Because the

			 * layout of the data when saving the sve state depends

			 * on the VL, we need to use a consistent (i.e., the

			 * maximum) VL.

			 * Note that this means that at guest exit ZCR_EL1 is

			 * not necessarily the same as on guest entry.

			 *

			 * Restoring the VL isn't needed in VHE mode since

			 * ZCR_EL2 (accessed via ZCR_EL1) would fulfill the same

			 * role when doing the save from EL2.

			 */

			if (!has_vhe())

				sve_cond_update_zcr_vq(vcpu_sve_max_vq(vcpu) - 1,

						       SYS_ZCR_EL1);

		}



		/*

		 * Flush (save and invalidate) the fpsimd/sve state so that if

		 * the host tries to use fpsimd/sve, it's not using stale data

		 * from the guest.

		 *

		 * Flushing the state sets the TIF_FOREIGN_FPSTATE bit for the

		 * context unconditionally, in both nVHE and VHE. This allows

		 * the kernel to restore the fpsimd/sve state, including ZCR_EL1

		 * when needed.

		 */

		fpsimd_save_and_flush_cpu_state();

	} else if (has_vhe() && system_supports_sve()) {

		/*