Re: [PATCH] KVM: arm64: Advertise ID_AA64PFR2_EL1.GCIE

[Nathan Chancellor <nathan@kernel.org>]

Hi Marc,

On Wed, Apr 01, 2026 at 06:00:17PM +0100, Marc Zyngier wrote:
> As we are missing ID_AA64PFR2_EL1.GCIE from the kernel feature set,
> userspace cannot write ID_AA64PFR2_EL1 with GCIE set, even if we are
> on a GICv5 host.
> 
> Add the required field description.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kernel/cpufeature.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 32c2dbcc0c641..5bca6e064ca72 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -327,6 +327,7 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr2[] = {
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_FPMR_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_MTEFAR_SHIFT, 4, ID_AA64PFR2_EL1_MTEFAR_NI),
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_MTESTOREONLY_SHIFT, 4, ID_AA64PFR2_EL1_MTESTOREONLY_NI),
> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_GCIE_SHIFT, 4, ID_AA64PFR2_EL1_GCIE_NI),
>  	ARM64_FTR_END,
>  };
>  
> -- 
> 2.47.3
> 

After this change in -next as commit 899ff451fcee ("KVM: arm64:
Advertise ID_AA64PFR2_EL1.GCIE"), I am seeing a warning on boot in my
simple QEMU boot tests.

  $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux- mrproper virtconfig Image.gz

  $ curl -LSs https://github.com/ClangBuiltLinux/boot-utils/releases/download/20241120-044434/arm64-rootfs.cpio.zst | zstd -d >rootfs.cpio

  $ qemu-system-aarch64 \
      -display none \
      -nodefaults \
      -machine virt,gic-version=max \
      -append 'console=ttyAMA0 earlycon' \
      -kernel arch/arm64/boot/Image.gz \
      -initrd rootfs.cpio \
      -cpu host \
      -enable-kvm \
      -m 1G \
      -smp 8 \
      -serial mon:stdio
  [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x413fd0c1]
  [    0.000000] Linux version 7.0.0-rc4-00058-g899ff451fcee (nathan@aadp) (aarch64-linux-gcc (GCC) 15.2.0, GNU ld (GNU Binutils) 2.45) #1 SMP PREEMPT Sat Apr  4 06:55:05 MST 2026
  ...
  [    0.000000] ------------[ cut here ]------------
  [    0.000000] SYS_ID_AA64PFR2_EL1 has feature overlap at shift 12
  [    0.000000] WARNING: arch/arm64/kernel/cpufeature.c:986 at init_cpu_features+0xbc/0x344, CPU#0: swapper/0
  [    0.000000] Modules linked in:
  [    0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.0.0-rc4-00058-g899ff451fcee #1 PREEMPT
  [    0.000000] Hardware name: linux,dummy-virt (DT)
  [    0.000000] pstate: 600000c5 (nZCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
  [    0.000000] pc : init_cpu_features+0xbc/0x344
  [    0.000000] lr : init_cpu_features+0xbc/0x344
  [    0.000000] sp : ffffcd0982373db0
  [    0.000000] x29: ffffcd0982373db0 x28: 0000000000000010 x27: ffffcd0981c63878
  [    0.000000] x26: 0000000000000018 x25: ffffcd0982013f38 x24: ffffcd0981c69068
  [    0.000000] x23: ffffcd0981c635f0 x22: ffffcd0982388640 x21: 0000000000000003
  [    0.000000] x20: 0000000000000017 x19: ffffcd09824c9308 x18: 000000000000000a
  [    0.000000] x17: 5d305b203837205d x16: 305b203737205d30 x15: 0000000000000000
  [    0.000000] x14: 0000000000000000 x13: 3231207466696873 x12: 2074612070616c72
  [    0.000000] x11: 0000000000000058 x10: 0000000000000018 x9 : ffffcd0982396598
  [    0.000000] x8 : 0000000000057fa8 x7 : 000000000000002a x6 : ffffcd09823ee598
  [    0.000000] x5 : ffffcd09823ee598 x4 : 0000000000000000 x3 : 0000000000000000
  [    0.000000] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffffcd09823852c0
  [    0.000000] Call trace:
  [    0.000000]  init_cpu_features+0xbc/0x344 (P)
  [    0.000000]  cpuinfo_store_boot_cpu+0x48/0x54
  [    0.000000]  smp_prepare_boot_cpu+0x28/0x38
  [    0.000000]  start_kernel+0x248/0x780
  [    0.000000]  __primary_switched+0x88/0x90
  [    0.000000] ---[ end trace 0000000000000000 ]---
  ...
  ```

Is this expected? I assume not, hence the report. If there is any
information I can provide or patches I can test, I am more than happy to
do so.

Cheers,
Nathan

# bad: [2febe6e6ee6e34c7754eff3c4d81aa7b0dcb7979] Add linux-next specific files for 20260403
# good: [d8a9a4b11a137909e306e50346148fc5c3b63f9d] Merge tag 'v7.0-rc6-smb3-client-fix' of git://git.samba.org/sfrench/cifs-2.6
git bisect start 'origin/master' 'origin/stable'
# bad: [dc9c72702de7e46679f8c2ab2175ab7070baa3ee] Merge branch 'main' of https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git
git bisect bad dc9c72702de7e46679f8c2ab2175ab7070baa3ee
# bad: [d5ea83fae6558b2431b022781f6ccb74d241b2bb] Merge branch 'for-next' of https://git.kernel.org/pub/scm/linux/kernel/git/s390/linux.git
git bisect bad d5ea83fae6558b2431b022781f6ccb74d241b2bb
# bad: [d16ebd2d0dc90f1561d0a44a828db2c9148ad5f4] Merge branch 'for-next/perf' of https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git
git bisect bad d16ebd2d0dc90f1561d0a44a828db2c9148ad5f4
# bad: [768784742008ccebbf3e88a26a6e9e6aad76dc8e] Merge branch 'timekeeping-next' of https://github.com/Rust-for-Linux/linux.git
git bisect bad 768784742008ccebbf3e88a26a6e9e6aad76dc8e
# good: [b671065be2bef906bb3e0c1bc7be4055e84ea1d3] Merge branch 'at91-fixes' of https://git.kernel.org/pub/scm/linux/kernel/git/at91/linux.git
git bisect good b671065be2bef906bb3e0c1bc7be4055e84ea1d3
# bad: [bd1f7328e25426f08cafa8333293411788c7957b] Merge branch kvm-arm64/vgic-fixes-7.1 into kvmarm-master/next
git bisect bad bd1f7328e25426f08cafa8333293411788c7957b
# good: [1cd0bb0425594cea9baf862393a4ca9cc0c018a3] Merge branch kvm-arm64/pkvm-psci into kvmarm-master/next
git bisect good 1cd0bb0425594cea9baf862393a4ca9cc0c018a3
# good: [bc20692f528b2ac8226bafe5b1db9a1f8be96dbf] KVM: arm64: Don't hold 'vm_table_lock' across guest page reclaim
git bisect good bc20692f528b2ac8226bafe5b1db9a1f8be96dbf
# good: [adb70b3a8b31e9eb05f2ec3c76d85f9a7a8c8cbc] KVM: arm64: Directly expose mapping prot and kill kvm_s2_fault
git bisect good adb70b3a8b31e9eb05f2ec3c76d85f9a7a8c8cbc
# good: [be46a408f376df31762e8a9914dc6d082755e686] KVM: arm64: Correctly plumb ID_AA64PFR2_EL1 into pkvm idreg handling
git bisect good be46a408f376df31762e8a9914dc6d082755e686
# good: [33cdd7f8fa32c92317aa521bd6f407a3cba8474b] Merge branch kvm-arm64/spe-trbe-nvhe into kvmarm-master/next
git bisect good 33cdd7f8fa32c92317aa521bd6f407a3cba8474b
# good: [e54971a0468a8bc82b1976d5b010392d7cb689b9] Merge branch kvm-arm64/vgic-fixes-7.1 into kvmarm-master/next
git bisect good e54971a0468a8bc82b1976d5b010392d7cb689b9
# bad: [899ff451fcee1289f3f37d061da66c3e38748a69] KVM: arm64: Advertise ID_AA64PFR2_EL1.GCIE
git bisect bad 899ff451fcee1289f3f37d061da66c3e38748a69
# good: [9c1ac77ddfc90b6292ef63a4fa5ab6f9e4b29981] KVM: arm64: vgic-v5: Fold PPI state for all exposed PPIs
git bisect good 9c1ac77ddfc90b6292ef63a4fa5ab6f9e4b29981
# first bad commit: [899ff451fcee1289f3f37d061da66c3e38748a69] KVM: arm64: Advertise ID_AA64PFR2_EL1.GCIE