[PATCH] KVM: arm64: Always populate FGT masks at boot time

[PATCH] KVM: arm64: Always populate FGT masks at boot time

[Marc Zyngier]

We currently only populate the FGT masks if the underlying HW does
support FEAT_FGT. However, with the addition of the RES1 support for
system registers, this results in a lot of noise at boot time, as
reported by Nathan.

That's because even if FGT isn't supported, we still check for the
attribution of the bits to particular features, and not keeping the
masks up-to-date leads to (fairly harmess) warnings.

Given that we want these checks to be enforced even if the HW doesn't
support FGT, enable the generation of FGT masks unconditionally (this
is rather cheap anyway). Only the storage of the FGT configuration is
avoided, which will save a tiny bit of memory on these machines.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Fixes: c259d763e6b0 ("KVM: arm64: Account for RES1 bits in DECLARE_FEAT_MAP() and co")
Link: https://lore.kernel.org/r/20260120211558.GA834868@ax162
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/emulate-nested.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
index 75d49f83342a5..e5874effdf167 100644
--- a/arch/arm64/kvm/emulate-nested.c
+++ b/arch/arm64/kvm/emulate-nested.c
@@ -2276,9 +2276,6 @@ int __init populate_nv_trap_config(void)
 	kvm_info("nv: %ld coarse grained trap handlers\n",
 		 ARRAY_SIZE(encoding_to_cgt));
 
-	if (!cpus_have_final_cap(ARM64_HAS_FGT))
-		goto check_mcb;
-
 	for (int i = 0; i < ARRAY_SIZE(encoding_to_fgt); i++) {
 		const struct encoding_to_trap_config *fgt = &encoding_to_fgt[i];
 		union trap_config tc;
@@ -2298,6 +2295,15 @@ int __init populate_nv_trap_config(void)
 			}
 
 			tc.val |= fgt->tc.val;
+
+			if (!aggregate_fgt(tc)) {
+				ret = -EINVAL;
+				print_nv_trap_error(fgt, "FGT bit is reserved", ret);
+			}
+
+			if (!cpus_have_final_cap(ARM64_HAS_FGT))
+				continue;
+
 			prev = xa_store(&sr_forward_xa, enc,
 					xa_mk_value(tc.val), GFP_KERNEL);
 
@@ -2305,11 +2311,6 @@ int __init populate_nv_trap_config(void)
 				ret = xa_err(prev);
 				print_nv_trap_error(fgt, "Failed FGT insertion", ret);
 			}
-
-			if (!aggregate_fgt(tc)) {
-				ret = -EINVAL;
-				print_nv_trap_error(fgt, "FGT bit is reserved", ret);
-			}
 		}
 	}
 
@@ -2325,7 +2326,6 @@ int __init populate_nv_trap_config(void)
 	kvm_info("nv: %ld fine grained trap handlers\n",
 		 ARRAY_SIZE(encoding_to_fgt));
 
-check_mcb:
 	for (int id = __MULTIPLE_CONTROL_BITS__; id < __COMPLEX_CONDITIONS__; id++) {
 		const enum cgt_group_id *cgids;
 
-- 
2.47.3

Re: [PATCH] KVM: arm64: Always populate FGT masks at boot time

[Marc Zyngier]

On Thu, 22 Jan 2026 08:51:53 +0000, Marc Zyngier wrote:
> We currently only populate the FGT masks if the underlying HW does
> support FEAT_FGT. However, with the addition of the RES1 support for
> system registers, this results in a lot of noise at boot time, as
> reported by Nathan.
> 
> That's because even if FGT isn't supported, we still check for the
> attribution of the bits to particular features, and not keeping the
> masks up-to-date leads to (fairly harmess) warnings.
> 
> [...]

Applied to next, thanks!

[1/1] KVM: arm64: Always populate FGT masks at boot time
      commit: 54adbfe40e3b6d238293cc36ef071ed9f35c8af7

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.