linux-kernel - [PATCH v2 13/45] KVM: arm64: Force guest EL1 to use user-space's partid configuration

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Message-ID: <20251219181147.3404071-14-ben.horgan@arm.com>
Date: Fri, 19 Dec 2025 18:11:15 +0000
From: Ben Horgan <ben.horgan@....com>
To: ben.horgan@....com
Cc: amitsinght@...vell.com,
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Subject: [PATCH v2 13/45] KVM: arm64: Force guest EL1 to use user-space's partid configuration

From: James Morse <james.morse@....com>

While we trap the guest's attempts to read/write the MPAM control
registers, the hardware continues to use them. Guest-EL0 uses KVM's
user-space's configuration, as the value is left in the register, and
guest-EL1 uses either the host kernel's configuration, or in the case of
VHE, the UNKNOWN reset value of MPAM1_EL1.

We want to force the guest-EL1 to use KVM's user-space's MPAM
configuration. On nVHE rely on MPAM0_EL1 and MPAM1_EL1 always being
programmed the same and on VHE copy MPAM0_EL1 into the guest's
MPAM1_EL1. There is no need to restore as this is out of context once TGE
is set.

Signed-off-by: James Morse <james.morse@....com>
Signed-off-by: Ben Horgan <ben.horgan@....com>
---
Changes since rfc:
Drop the unneeded __mpam_guest_load() in nvhre and the MPAM1_EL1 save restore
Defer EL2 handling until next patch
---
 arch/arm64/kvm/hyp/vhe/sysreg-sr.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/arm64/kvm/hyp/vhe/sysreg-sr.c b/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
index f28c6cf4fe1b..a2e8f6187675 100644
--- a/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
+++ b/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
@@ -183,6 +183,16 @@ void sysreg_restore_guest_state_vhe(struct kvm_cpu_context *ctxt)
 }
 NOKPROBE_SYMBOL(sysreg_restore_guest_state_vhe);
 
+/*
+ * The _EL0 value was written by the host's context switch and belongs to the
+ * VMM. Copy this into the guest's _EL1 register.
+ */
+static inline void __mpam_guest_load(void)
+{
+	if (system_supports_mpam())
+		write_sysreg_el1(read_sysreg_s(SYS_MPAM0_EL1), SYS_MPAM1);
+}
+
 /**
  * __vcpu_load_switch_sysregs - Load guest system registers to the physical CPU
  *
@@ -222,6 +232,7 @@ void __vcpu_load_switch_sysregs(struct kvm_vcpu *vcpu)
 	 */
 	__sysreg32_restore_state(vcpu);
 	__sysreg_restore_user_state(guest_ctxt);
+	__mpam_guest_load();
 
 	if (unlikely(is_hyp_ctxt(vcpu))) {
 		__sysreg_restore_vel2_state(vcpu);
-- 
2.43.0