[PATCH] KVM: arm64: Trap guest MPAM accesses whenever MPAM is implemented

[Fuad Tabba <fuad.tabba@linux.dev>]

finalise_el2_state() clears the EL2 MPAM traps whenever the ID registers
advertise MPAM, but KVM sets them only when ARM64_MPAM is set, which
also requires MPAMEN. Without EL3 the enable is EL2's own and nothing
sets it, so the cap stays off and a guest reaches the MPAM registers
while ID_AA64PFR0_EL1.MPAM reads 0 for it.

Gate the traps on the ID registers alone. MPAMEN is not a term in any
MPAM accessor, so they take effect without it. finalise_el2_state
already wrote MPAM2_EL2 under the same condition, so MPAM3_EL3.TRAPLOWER
is clear wherever the cap is set, and arm64.nompam still clears it.

Fixes: 31ff96c38ea3 ("KVM: arm64: Fix missing traps of guest accesses to the MPAM registers")
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
Found this while working on the other MPAM thread [1].

[1] https://lore.kernel.org/all/CA+EHjTxeWxZiuSmnKLGLxTBXP4oJT7-LuffbPAyCSZZ5TW=5Ew@mail.gmail.com/

 arch/arm64/include/asm/cpufeature.h     |  5 +++++
 arch/arm64/kernel/cpufeature.c          | 13 +++++++++++++
 arch/arm64/kvm/hyp/include/hyp/switch.h |  4 ++--
 arch/arm64/tools/cpucaps                |  1 +
 4 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 7404a6e83a930..8863ae99596bc 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -873,6 +873,11 @@ static __always_inline bool system_supports_mpam_hcr(void)
 	return alternative_has_cap_unlikely(ARM64_MPAM_HCR);
 }
 
+static __always_inline bool system_supports_mpam_sysregs(void)
+{
+	return alternative_has_cap_unlikely(ARM64_MPAM_SYSREGS);
+}
+
 static inline bool system_supports_pmuv3(void)
 {
 	return cpus_have_final_cap(ARM64_HAS_PMUV3);
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 17b83a2518a8f..36a27692e5cf7 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2501,6 +2501,13 @@ test_has_mpam(const struct arm64_cpu_capabilities *entry, int scope)
 	return (read_sysreg_s(SYS_MPAM1_EL1) & MPAM1_EL1_MPAMEN);
 }
 
+static bool
+test_has_mpam_sysregs(const struct arm64_cpu_capabilities *entry, int __unused)
+{
+	/* The registers exist whether or not firmware enabled MPAM. */
+	return detect_ftr_has_mpam();
+}
+
 static void
 cpu_enable_mpam(const struct arm64_cpu_capabilities *entry)
 {
@@ -3116,6 +3123,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.matches = test_has_mpam,
 		.cpu_enable = cpu_enable_mpam,
 	},
+	{
+		.desc = "Memory Partitioning And Monitoring system registers",
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.capability = ARM64_MPAM_SYSREGS,
+		.matches = test_has_mpam_sysregs,
+	},
 	{
 		.desc = "Memory Partitioning And Monitoring Virtualisation",
 		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index 1ce7130e25490..8941335724f6b 100644
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -298,7 +298,7 @@ static inline void  __activate_traps_mpam(struct kvm_vcpu *vcpu)
 	u64 clr = MPAM2_EL2_EnMPAMSM;
 	u64 set = MPAM2_EL2_TRAPMPAM0EL1 | MPAM2_EL2_TRAPMPAM1EL1;
 
-	if (!system_supports_mpam())
+	if (!system_supports_mpam_sysregs())
 		return;
 
 	/* trap guest access to MPAMIDR_EL1 */
@@ -317,7 +317,7 @@ static inline void __deactivate_traps_mpam(void)
 	u64 clr = MPAM2_EL2_TRAPMPAM0EL1 | MPAM2_EL2_TRAPMPAM1EL1 | MPAM2_EL2_TIDR;
 	u64 set = MPAM2_EL2_EnMPAMSM;
 
-	if (!system_supports_mpam())
+	if (!system_supports_mpam_sysregs())
 		return;
 
 	sysreg_clear_set_s(SYS_MPAM2_EL2, clr, set);
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index 2775ba3359cfe..aa5be51385f68 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -78,6 +78,7 @@ KVM_PROTECTED_MODE
 MISMATCHED_CACHE_TYPE
 MPAM
 MPAM_HCR
+MPAM_SYSREGS
 MTE
 MTE_ASYMM
 MTE_FAR

base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
-- 
2.39.5