Re: [PATCH v4 0/5] KVM: arm64: Writable MIDR/REVIDR (and associated baggage)

[Marc Zyngier <maz@kernel.org>]

On Tue, 25 Feb 2025 00:53:56 +0000,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> This is hopefully the last take on the writable MIDR/REVIDR/AIDR series.
> 
> I've restructured things in order to work around the existing
> shortcomings, have a stable-worthy backport for SMIDR_EL1, and avoid
> breaking ABI.
> 
> I personally hate using a capability for this, but given the subtlety of
> the issue (and our habit of breaking save/restore) it is probably best
> to put userspace in the driver's seat.
> 
> v3: https://lore.kernel.org/kvmarm/20250218163443.32836-1-sebott@redhat.com/
> 
> v3 -> v4:
>  - Trap SMIDR_EL1 accesses so they actually UNDEF
>  - Snapshot MIDR, REVIDR, and AIDR from the boot CPU
>  - Require buy-in from userspace to make ID registers writable

Here's what I added to the series in order to handle AIDR/REVIDR. Not
exactly pretty, but it does the job (tested on my Synquacker running a
hand-crafted guest).

Feel free to squash it into patch #1.

Thanks,

	M.

From: Marc Zyngier <maz@kernel.org>
Date: Tue, 25 Feb 2025 13:46:55 +0000
Subject: [PATCH] KVM: arm64: Map AArch32 AIDR/REVIDR to their AArch64
 counterpart

In order to handle AIDR/REVIDR traps from an AArch32 guest, add
some ugly handling to kvm_handle_cp15_32().

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/sys_regs.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index c1727fe6c8cb0..1ceaec61796c9 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -4398,9 +4398,13 @@ int kvm_handle_cp15_32(struct kvm_vcpu *vcpu)
 	 * Certain AArch32 ID registers are handled by rerouting to the AArch64
 	 * system register table. Registers in the ID range where CRm=0 are
 	 * excluded from this scheme as they do not trivially map into AArch64
-	 * system register encodings.
+	 * system register encodings, except for AIDR/REVIDR.
 	 */
-	if (params.Op1 == 0 && params.CRn == 0 && params.CRm)
+	if (params.Op1 == 0 && params.CRn == 0 &&
+	    (params.CRm || params.Op2 == 6 /* REVIDR */))
+		return kvm_emulate_cp15_id_reg(vcpu, &params);
+	if (params.Op1 == 1 && params.CRn == 0 &&
+	    params.CRm == 0 && params.Op2 == 7 /* AIDR */)
 		return kvm_emulate_cp15_id_reg(vcpu, &params);
 
 	return kvm_handle_cp_32(vcpu, &params, cp15_regs, ARRAY_SIZE(cp15_regs));
-- 
2.39.2


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