[PATCH v3] KVM: arm64: Fix confusion in documentation for pKVM SME assert

[Mark Brown]

As raised in the review comments for the original patch the assert and
comment added in afb91f5f8ad7 ("KVM: arm64: Ensure that SME controls are
disabled in protected mode") are bogus. The comments says that we check
that we do not have SME enabled for a pKVM guest but the assert actually
checks to see if the host has anything set in SVCR which is unrelated to
the guest features or state, regardless of if those guests are protected
or not. This check is also made in the hypervisor, it will refuse to run
a guest if the check fails, so it appears that the assert here is
intended to improve diagnostics.

Update the comment to reflect the check in the code, and to clarify that
we do actually enforce this in the hypervisor. While we're here also
update to use a WARN_ON_ONCE() to avoid log spam if this triggers.

Fixes: afb91f5f8ad7 ("KVM: arm64: Ensure that SME controls are disabled in protected mode")
Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>
Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
---
Changes in v3:
- Rebase onto v6.11-rc1.
- Link to v2: https://lore.kernel.org/r/20240605-kvm-arm64-sme-assert-v2-1-54391b0032f4@xxxxxxxxxx

Changes in v2:
- Commit message tweaks.
- Change the assert to WARN_ON_ONCE().
- Link to v1: https://lore.kernel.org/r/20240604-kvm-arm64-sme-assert-v1-1-5d98348d00f8@xxxxxxxxxx
---
 arch/arm64/kvm/fpsimd.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index c53e5b14038d..8eca6b338bb2 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -92,11 +92,14 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 	}
 
 	/*
-	 * If normal guests gain SME support, maintain this behavior for pKVM
-	 * guests, which don't support SME.
+	 * The pKVM hypervisor does not yet understand how to save or
+	 * restore SME state for the host so double check that if we
+	 * are running with pKVM we have disabled SME.  The hypervisor
+	 * enforces this when the guest is run, this check is for
+	 * clearer diagnostics.
 	 */
-	WARN_ON(is_protected_kvm_enabled() && system_supports_sme() &&
-		read_sysreg_s(SYS_SVCR));
+	WARN_ON_ONCE(is_protected_kvm_enabled() && system_supports_sme() &&
+		     read_sysreg_s(SYS_SVCR));
 }
 
 /*

---
base-commit: 8400291e289ee6b2bf9779ff1c83a291501f017b
change-id: 20240604-kvm-arm64-sme-assert-5ad755d4e8a6

Best regards,
-- 
Mark Brown <broonie@xxxxxxxxxx>