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

[Marc Zyngier <maz@kernel.org>]

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.