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From: Oliver Upton To: Shameer Kolothum 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@huawei.com, wangzhou1@hisilicon.com, jiangkunkun@huawei.com, jonathan.cameron@huawei.com, anthony.jebson@huawei.com, linux-arm-kernel@lists.infradead.org, linuxarm@huawei.com Subject: Re: [PATCH v6 3/4] KVM: arm64: Report all the KVM/arm64-specific hypercalls Message-ID: References: <20250205132222.55816-1-shameerali.kolothum.thodi@huawei.com> <20250205132222.55816-4-shameerali.kolothum.thodi@huawei.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT On Fri, Feb 07, 2025 at 10:21:13AM -0800, Oliver Upton wrote: > On Wed, Feb 05, 2025 at 01:22:21PM +0000, Shameer Kolothum wrote: > > Currently ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID returns the > > bitmap corresponding to KVM_REG_ARM_VENDOR_HYP_BMAP and it only > > returns _KVM_FEATURES_FUNC_ID and _KVM_PTP_FUNC_ID. Change that > > to return all the KVM/arm64-specific hypercalls exposed by > > KVM/arm64 to guest operating systems. > > > > Signed-off-by: Shameer Kolothum > > --- > > 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) > > 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), > > + (ARM_SMCCC_KVM_FUNC_DISCOVER_IMPL_VER - 64)); > > 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. -- Thanks, Oliver