[PATCH] KVM: arm64: nv: Return correct RES0 bits for FGT registers

[PATCH] KVM: arm64: nv: Return correct RES0 bits for FGT registers

[Zenghui Yu]

From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>

We had extended the sysreg masking infrastructure to more general
registers, instead of restricting it to VNCR-backed registers, since
commit a0162020095e ("KVM: arm64: Extend masking facility to arbitrary
registers"). Fix kvm_get_sysreg_res0() to reflect this fact.

Note that we're sure that we only deal with FGT registers in
kvm_get_sysreg_res0(), the

	if (sr < __VNCR_START__)

is actually a never false, which should probably be removed later.

Fixes: 69c19e047dfe ("KVM: arm64: Add TCR2_EL2 to the sysreg arrays")
Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
---
 arch/arm64/kvm/emulate-nested.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
index 834f13fb1fb7..2d04fb56746e 100644
--- a/arch/arm64/kvm/emulate-nested.c
+++ b/arch/arm64/kvm/emulate-nested.c
@@ -2428,7 +2428,7 @@ static u64 kvm_get_sysreg_res0(struct kvm *kvm, enum vcpu_sysreg sr)
 
 	masks = kvm->arch.sysreg_masks;
 
-	return masks->mask[sr - __VNCR_START__].res0;
+	return masks->mask[sr - __SANITISED_REG_START__].res0;
 }
 
 static bool check_fgt_bit(struct kvm_vcpu *vcpu, enum vcpu_sysreg sr,
-- 
2.34.1

Re: [PATCH] KVM: arm64: nv: Return correct RES0 bits for FGT registers

[Marc Zyngier]

On Wed, 21 Jan 2026 10:16:31 +0000,
Zenghui Yu <zenghui.yu@linux.dev> wrote:
> 
> From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
> 
> We had extended the sysreg masking infrastructure to more general
> registers, instead of restricting it to VNCR-backed registers, since
> commit a0162020095e ("KVM: arm64: Extend masking facility to arbitrary
> registers"). Fix kvm_get_sysreg_res0() to reflect this fact.

Huh, what a howler! Thanks a lot for spotting this!

> 
> Note that we're sure that we only deal with FGT registers in
> kvm_get_sysreg_res0(), the
> 
> 	if (sr < __VNCR_START__)
> 
> is actually a never false, which should probably be removed later.

I think the biggest mistake is to have several places where we perform
sysreg index to array index conversions. We should try to unify that.

> 
> Fixes: 69c19e047dfe ("KVM: arm64: Add TCR2_EL2 to the sysreg arrays")
> Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>

Cc: stable@vger.kernel.org

Thanks again,

	M.

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

Re: [PATCH] KVM: arm64: nv: Return correct RES0 bits for FGT registers

[Marc Zyngier]

On Wed, 21 Jan 2026 18:16:31 +0800, Zenghui Yu wrote:
> We had extended the sysreg masking infrastructure to more general
> registers, instead of restricting it to VNCR-backed registers, since
> commit a0162020095e ("KVM: arm64: Extend masking facility to arbitrary
> registers"). Fix kvm_get_sysreg_res0() to reflect this fact.
> 
> Note that we're sure that we only deal with FGT registers in
> kvm_get_sysreg_res0(), the
> 
> [...]

Applied to next, thanks!

[1/1] KVM: arm64: nv: Return correct RES0 bits for FGT registers
      commit: 2eb80a2eee18762a33aa770d742d64fe47852c7e

Cheers,

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