RE: [PATCH v5 3/4] KVM: arm64: Report all the KVM/arm64-specific hypercalls

-----Original Message-----
From: Oliver Upton <redacted>
Sent: Monday, January 27, 2025 5:06 PM
To: Shameerali Kolothum Thodi <redacted>
Cc: kvmarm@lists.linux.dev; maz@kernel.org; catalin.marinas@arm.com;
will@kernel.org; mark.rutland@arm.com; cohuck@redhat.com;
eric.auger@redhat.com; sebott@redhat.com; yuzenghui
[off-list ref]; Wangzhou (B) [off-list ref];
jiangkunkun [off-list ref]; Jonathan Cameron
[off-list ref]; Anthony Jebson
[off-list ref]; linux-arm-kernel@lists.infradead.org;
Linuxarm [off-list ref]
Subject: Re: [PATCH v5 3/4] KVM: arm64: Report all the KVM/arm64-specific
hypercalls

Hi Shameer,

On Fri, Jan 24, 2025 at 03:17:31PM +0000, Shameer Kolothum wrote:

quoted

Currently ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID returns

the bitmap

quoted

corresponding to KVM_REG_ARM_VENDOR_HYP_BMAP and it only

returns

quoted

_KVM_FEATURES_FUNC_ID and _KVM_PTP_FUNC_ID. Change that to

return all

quoted

the KVM/arm64-specific hypercalls exposed by
KVM/arm64 to guest operating systems.

Signed-off-by: Shameer Kolothum

[off-list ref]

quoted

---
 arch/arm64/kvm/hypercalls.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index 27ce4cb44904..6132cb542200 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c

@@ -359,7 +359,11 @@ int kvm_smccc_call_handler(struct kvm_vcpu

*vcpu)

quoted

 		val[3] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3;
 		break;
 	case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
-		val[0] = smccc_feat->vendor_hyp_bmap;
+		val[0] = GENMASK(ARM_SMCCC_KVM_FUNC_MMIO_GUARD,
+				 ARM_SMCCC_KVM_FUNC_FEATURES);
+		/* Function numbers 8-63 are reserved for pKVM for now */
+		val[2] =

GENMASK((ARM_SMCCC_KVM_FUNC_DISCOVER_IMPL_CPUS - 64),

quoted

+

(ARM_SMCCC_KVM_FUNC_DISCOVER_IMPL_VER - 64));

quoted

 		break;

This isn't right. The pKVM carveout exists for some KVM-internal
bookkeeping to (hopefully) avoid breaking guest ABI between what's in the
downstream android kernel and what eventually gets accepted upstream.

The purpose of this hypercall is for the guest to discover what interfaces are
actually implemented by the hypervisor, and we definitely do not
implement these upstream at the moment.

Ok. I had some confusion on pKVM ones as I could see the kvm_arm_hyp_service_available()
being called with those in arm-pkvm-guest.c. So that explains it.
.
I will remove the pKVM ones and just add support to report the ones introduced
in this series.

Thanks,
Shameer