[PATCH] KVM: arm64: Hide unsupported MPAM from the guest

[James Morse]

Commit 011e5f5bf529 ("arm64/cpufeature: Add remaining feature bits in
ID_AA64PFR0 register") proactively added published features to the
cpufeature id registers.

If the platform supports these features, they are visible in the
sanitised ID registers that are exposed to KVM guests. This is a
problem as KVM doesn't support MPAM.

The hardware reset behaviour of MPAM is to be disabled at EL3. It
is unlikely anyone would ship a platform without firmware support,
the necessary initialisation has been upstream in the TF-A project
for over a year.

Firmware configures the EL2 registers to trap EL1 and EL0 access
to EL2. As KVM doesn't support MPAM, it doesn't change these
registers. Booting an MPAM capable kernel as a guest of mainline
causes KVM to take an unknown trap from an EL1 guest, and inject
an undef in response:
host:
| kvm [126]: Unsupported guest sys_reg access at: ffff800010093f24 [00000005]
|  { Op0( 3), Op1( 0), CRn(10), CRm( 5), Op2( 0), func_read },

guest:
| ------------[ cut here ]------------
| kernel BUG at arch/arm64/kernel/traps.c:409!
| Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
| Modules linked in:
| CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc1-00152-g570fa7e2d2ad #11605
| Hardware name: linux,dummy-virt (DT)
| pstate: 00000005 (nzcv daif -PAN -UAO)
| pc : do_undefinstr+0x2ec/0x310
| lr : do_undefinstr+0x2f8/0x310
...

This is a tad unfair on the guest as KVM said it supported the
feature. Mask out the MPAM feature.

Fixes: 011e5f5bf529 ("arm64/cpufeature: Add remaining feature bits in
ID_AA64PFR0 register")
Cc: <stable at vger.kernel.org>
Cc: Anshuman Khandual <anshuman.khandual at arm.com>
Signed-off-by: James Morse <james.morse at arm.com>

---
I'll be back at rc1 with the minimal KVM support to ensure the traps
are enabled and handled islently.
---
 arch/arm64/kvm/sys_regs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 077293b5115f..f736791f37ca 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1131,6 +1131,7 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
 		if (!vcpu_has_sve(vcpu))
 			val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
 		val &= ~(0xfUL << ID_AA64PFR0_AMU_SHIFT);
+		val &= ~(0xfUL << ID_AA64PFR0_MPAM_SHIFT);
 	} else if (id == SYS_ID_AA64ISAR1_EL1 && !vcpu_has_ptrauth(vcpu)) {
 		val &= ~((0xfUL << ID_AA64ISAR1_APA_SHIFT) |
 			 (0xfUL << ID_AA64ISAR1_API_SHIFT) |
-- 
2.28.0