-----Original Message-----
From: Oliver Upton <redacted>
Sent: Friday, February 7, 2025 6:24 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 v6 3/4] KVM: arm64: Report all the KVM/arm64-specific
hypercalls

On Fri, Feb 07, 2025 at 10:21:13AM -0800, Oliver Upton wrote:

quoted

On Wed, Feb 05, 2025 at 01:22:21PM +0000, Shameer Kolothum wrote:

quoted

Currently ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID returns

the

quoted

quoted

bitmap corresponding to KVM_REG_ARM_VENDOR_HYP_BMAP and it

only

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quoted

returns _KVM_FEATURES_FUNC_ID and _KVM_PTP_FUNC_ID. Change

that

quoted

quoted

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

Signed-off-by: Shameer Kolothum

[off-list ref]

quoted

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..5cef2590ffdf 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

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_PTP,
+				 ARM_SMCCC_KVM_FUNC_FEATURES);
+		/* Function numbers 2-63 are reserved for pKVM for now */
+		val[2] =

GENMASK((ARM_SMCCC_KVM_FUNC_DISCOVER_IMPL_CPUS - 64),

quoted

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+

(ARM_SMCCC_KVM_FUNC_DISCOVER_IMPL_VER - 64));

quoted

quoted

 		break;

This isn't right, vendor_hyp_bmap is very much load bearing. We have a
documented UAPI that allows userspace to control the hypercalls exposed
to the guest.

The idea being a user wants kernel rollback safety and doesn't expose
hypercalls that could potentially be revoked.

https://docs.kernel.org/virt/kvm/arm/fw-pseudo-registers.html#bitmap-

feature-firmware-registers

To add:

KVM cannot advertise the DISCOVER_IMPL* stuff unconditionally, since the
expectation is that userspace implements these hypercalls. These bits
may need to be writable from userspace but have a reset value of 0.

Ok. So IIUC, vendor_hyp_bmap actually holds the information which Vendor Hyp
services  are available to the user space and can be get/set using GET/SET _ONE_REG
interfaces.

Currently this bitmap is a 64 bit one and if we have to have a one to one mapping
between these bitmap and the hypercall function numbers, then that requires
some changes.

Because function numbers 2-63 are now reserved for pKVM and the new ones
introduced in this series take 64 & 65.

May be we can have KVM_REG_ARM_VENDOR_HYP_BMAP_2  which represents
64-127?

Or can we take the next available bits(2 & 3) for KVM_REG_ARM_VENDOR_HYP_BMAP
and then map it to the function number appropriately (64 & 65) when
ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID is handled?

Thoughts?

Thanks,
Shameer