Re: [PATCH v2 08/13] KVM: arm64: Add support for KVM_EXIT_HYPERCALL

Marc Zyngier
2023-03-31T17:11:01Z

On Thu, 30 Mar 2023 16:49:13 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
>
> In anticipation of user hypercall filters, add the necessary plumbing to
> get SMCCC calls out to userspace. Even though the exit structure has
> space for KVM to pass register arguments, let's just avoid it altogether
> and let userspace poke at the registers via KVM_GET_ONE_REG.
>
> This deliberately stretches the definition of a 'hypercall' to cover
> SMCs from EL1 in addition to the HVCs we know and love. KVM doesn't
> support EL1 calls into secure services, but now we can paint that as a
> userspace problem and be done with it.
>
> Finally, we need a flag to let userspace know what conduit instruction
> was used (i.e. SMC vs. HVC).
>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
> Documentation/virt/kvm/api.rst | 22 ++++++++++++++++++++--
> arch/arm64/include/uapi/asm/kvm.h | 4 ++++
> arch/arm64/kvm/handle_exit.c | 4 +++-
> arch/arm64/kvm/hypercalls.c | 17 +++++++++++++++++
> 4 files changed, 44 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 9b01e3d0e757..c8ab2f730945 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6221,11 +6221,29 @@ to the byte array.
> __u64 flags;
> } hypercall;
>
> -Unused. This was once used for 'hypercall to userspace'. To implement
> -such functionality, use KVM_EXIT_IO (x86) or KVM_EXIT_MMIO (all except s390).
> +
> +It is strongly recommended that userspace use ``KVM_EXIT_IO`` (x86) or
> +``KVM_EXIT_MMIO`` (all except s390) to implement functionality that
> +requires a guest to interact with host userpace.
>
> .. note:: KVM_EXIT_IO is significantly faster than KVM_EXIT_MMIO.
>
> +For arm64:
> +----------
> +
> +SMCCC exits can be enabled depending on the configuration of the SMCCC
> +filter. See the Documentation/virt/kvm/devices/vm.rst
> +``KVM_ARM_SMCCC_FILTER`` for more details.

Maybe this hunk should come with the following patch which actually
adds that doc.

> +
> +``nr`` contains the function ID of the guest's SMCCC call. Userspace is
> +expected to use the ``KVM_GET_ONE_REG`` ioctl to retrieve the call
> +parameters from the vCPU's GPRs.
> +
> +Definition of ``flags``:
> + - ``KVM_HYPERCALL_EXIT_SMC``: Indicates that the guest used the SMC
> + conduit to initiate the SMCCC call. If this bit is 0 then the guest
> + used the HVC conduit for the SMCCC call.
> +
> ::
>
> /* KVM_EXIT_TPR_ACCESS */
> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index bbab92402510..1dabb7d05514 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -472,12 +472,16 @@ enum {
> enum kvm_smccc_filter_action {
> KVM_SMCCC_FILTER_ALLOW = 0,
> KVM_SMCCC_FILTER_DENY,
> + KVM_SMCCC_FILTER_FWD_TO_USER,
>
> #ifdef __KERNEL__
> NR_SMCCC_FILTER_ACTIONS
> #endif
> };
>
> +/* arm64-specific KVM_EXIT_HYPERCALL flags */
> +#define KVM_HYPERCALL_EXIT_SMC (1U << 0)
> +
> #endif
>
> #endif /* __ARM_KVM_H__ */
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 68f95dcd41a1..3f43e20c48b6 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -71,7 +71,9 @@ static int handle_smc(struct kvm_vcpu *vcpu)
> * Trap exception, not a Secure Monitor Call exception [...]"
> *
> * We need to advance the PC after the trap, as it would
> - * otherwise return to the same address...
> + * otherwise return to the same address. Furthermore, pre-incrementing
> + * the PC before potentially exiting to userspace maintains the same
> + * abstraction for both SMCs and HVCs.
> */
> kvm_incr_pc(vcpu);
>
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index 73b218ddd1a5..7e8c850847c1 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -180,6 +180,19 @@ static u8 kvm_smccc_get_action(struct kvm_vcpu *vcpu, u32 func_id)
> return KVM_SMCCC_FILTER_DENY;
> }
>
> +static void kvm_prepare_hypercall_exit(struct kvm_vcpu *vcpu, u32 func_id)
> +{
> + u8 ec = ESR_ELx_EC(kvm_vcpu_get_esr(vcpu));
> + struct kvm_run *run = vcpu->run;
> +
> + run->exit_reason = KVM_EXIT_HYPERCALL;
> + run->hypercall.nr = func_id;
> + run->hypercall.flags = 0;
> +
> + if (ec == ESR_ELx_EC_SMC32 || ec == ESR_ELx_EC_SMC64)
> + run->hypercall.flags |= KVM_HYPERCALL_EXIT_SMC;
> +}
> +
> int kvm_smccc_call_handler(struct kvm_vcpu *vcpu)
> {
> struct kvm_smccc_features *smccc_feat = &vcpu->kvm->arch.smccc_feat;
> @@ -192,6 +205,10 @@ int kvm_smccc_call_handler(struct kvm_vcpu *vcpu)
> action = kvm_smccc_get_action(vcpu, func_id);
> if (action == KVM_SMCCC_FILTER_DENY)
> goto out;
> + if (action == KVM_SMCCC_FILTER_FWD_TO_USER) {
> + kvm_prepare_hypercall_exit(vcpu, func_id);
> + return 0;
> + }

nit: maybe write this as a switch statement?

>
> switch (func_id) {
> case ARM_SMCCC_VERSION_FUNC_ID:

Thanks,

M.

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