Re: [RFC PATCH 5/5] KVM: arm64: Pass PSCI calls to userspace

[Fuad Tabba]

Hi Jean-Philippe,


On Tue, Jun 8, 2021 at 4:54 PM Jean-Philippe Brucker
<jean-philippe@xxxxxxxxxx> wrote:
>
> Let userspace request to handle PSCI calls, by setting the new
> KVM_CAP_ARM_PSCI_TO_USER capability.
>
> SMCCC probe requires PSCI v1.x. If userspace only implements PSCI v0.2,
> the guest won't query SMCCC support through PSCI and won't use the
> spectre workarounds. We could hijack PSCI_VERSION and pretend to support
> v1.0 if userspace does not, then handle all v1.0 calls ourselves
> (including guessing the PSCI feature set implemented by the guest), but
> that seems unnecessary. After all the API already allows userspace to
> force a version lower than v1.0 using the firmware pseudo-registers.
>
> The KVM_REG_ARM_PSCI_VERSION pseudo-register currently resets to either
> v0.1 if userspace doesn't set KVM_ARM_VCPU_PSCI_0_2, or
> KVM_ARM_PSCI_LATEST (1.0).
>
> Suggested-by: James Morse <james.morse@xxxxxxx>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> ---
>  Documentation/virt/kvm/api.rst      | 14 ++++++++++++++
>  Documentation/virt/kvm/arm/psci.rst |  1 +
>  arch/arm64/include/asm/kvm_host.h   |  1 +
>  include/kvm/arm_hypercalls.h        |  1 +
>  include/uapi/linux/kvm.h            |  1 +
>  arch/arm64/kvm/arm.c                | 10 +++++++---
>  arch/arm64/kvm/hypercalls.c         |  2 +-
>  arch/arm64/kvm/psci.c               | 13 +++++++++++++
>  8 files changed, 39 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 3d8c1661e7b2..f24eb70e575d 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6907,3 +6907,17 @@ available to the guest on migration.
>  This capability indicates that KVM can pass unhandled hypercalls to userspace,
>  if the VMM enables it. Hypercalls are passed with KVM_EXIT_HYPERCALL in
>  kvm_run::hypercall.
> +
> +8.34 KVM_CAP_ARM_PSCI_TO_USER
> +-----------------------------
> +
> +:Architectures: arm64
> +
> +When the VMM enables this capability, all PSCI calls are passed to userspace
> +instead of being handled by KVM. Capability KVM_CAP_ARM_HVC_TO_USER must be
> +enabled first.
> +
> +Userspace should support at least PSCI v1.0. Otherwise SMCCC features won't be
> +available to the guest. Userspace does not need to handle the SMCCC_VERSION
> +parameter for the PSCI_FEATURES function. The KVM_ARM_VCPU_PSCI_0_2 vCPU
> +feature should be set even if this capability is enabled.
> diff --git a/Documentation/virt/kvm/arm/psci.rst b/Documentation/virt/kvm/arm/psci.rst
> index d52c2e83b5b8..110011d1fa3f 100644
> --- a/Documentation/virt/kvm/arm/psci.rst
> +++ b/Documentation/virt/kvm/arm/psci.rst
> @@ -34,6 +34,7 @@ The following register is defined:
>    - Allows any PSCI version implemented by KVM and compatible with
>      v0.2 to be set with SET_ONE_REG
>    - Affects the whole VM (even if the register view is per-vcpu)
> +  - Defaults to PSCI 1.0 if userspace enables KVM_CAP_ARM_PSCI_TO_USER.

Should this be "to PSCI 1.x", to match the comment and for future expansion?

>
>  * KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1:
>      Holds the state of the firmware support to mitigate CVE-2017-5715, as
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 25554ce97045..5d74b769c16d 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -124,6 +124,7 @@ struct kvm_arch {
>          */
>         bool return_nisv_io_abort_to_user;
>         bool hvc_to_user;
> +       bool psci_to_user;
>
>         /*
>          * VM-wide PMU filter, implemented as a bitmap and big enough for
> diff --git a/include/kvm/arm_hypercalls.h b/include/kvm/arm_hypercalls.h
> index 0e2509d27910..b66c6a000ef3 100644
> --- a/include/kvm/arm_hypercalls.h
> +++ b/include/kvm/arm_hypercalls.h
> @@ -6,6 +6,7 @@
>
>  #include <asm/kvm_emulate.h>
>
> +int kvm_hvc_user(struct kvm_vcpu *vcpu);
>  int kvm_hvc_call_handler(struct kvm_vcpu *vcpu);
>
>  static inline u32 smccc_get_function(struct kvm_vcpu *vcpu)
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index aa831986a399..2b8e55aa7e1e 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1085,6 +1085,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_PTP_KVM 198
>  #define KVM_CAP_ARM_MP_HALTED 199
>  #define KVM_CAP_ARM_HVC_TO_USER 200
> +#define KVM_CAP_ARM_PSCI_TO_USER 201
>
>  #ifdef KVM_CAP_IRQ_ROUTING
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 074197721e97..bc3e63b0b3ad 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -83,7 +83,7 @@ int kvm_arch_check_processor_compat(void *opaque)
>  int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>                             struct kvm_enable_cap *cap)
>  {
> -       int r;
> +       int r = -EINVAL;
>
>         if (cap->flags)
>                 return -EINVAL;
> @@ -97,8 +97,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>                 r = 0;
>                 kvm->arch.hvc_to_user = true;
>                 break;
> -       default:
> -               r = -EINVAL;
> +       case KVM_CAP_ARM_PSCI_TO_USER:
> +               if (kvm->arch.hvc_to_user) {
> +                       r = 0;
> +                       kvm->arch.psci_to_user = true;
> +               }

Should this return -EINVAL if hvc_to_user isn't set, rather than
silently not setting psci_to_user, or should it be a parameter of
KVM_CAP_ARM_HVC_TO_USER rather than its own thing?

Thanks,
/fuad


>                 break;
>         }
>
> @@ -213,6 +216,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>         case KVM_CAP_PTP_KVM:
>         case KVM_CAP_ARM_MP_HALTED:
>         case KVM_CAP_ARM_HVC_TO_USER:
> +       case KVM_CAP_ARM_PSCI_TO_USER:
>                 r = 1;
>                 break;
>         case KVM_CAP_SET_GUEST_DEBUG2:
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index ccc2015eddf9..621d5a5b7e48 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -58,7 +58,7 @@ static void kvm_ptp_get_time(struct kvm_vcpu *vcpu, u64 *val)
>         val[3] = lower_32_bits(cycles);
>  }
>
> -static int kvm_hvc_user(struct kvm_vcpu *vcpu)
> +int kvm_hvc_user(struct kvm_vcpu *vcpu)
>  {
>         int i;
>         struct kvm_run *run = vcpu->run;
> diff --git a/arch/arm64/kvm/psci.c b/arch/arm64/kvm/psci.c
> index 42a307ceb95f..7f44ee527966 100644
> --- a/arch/arm64/kvm/psci.c
> +++ b/arch/arm64/kvm/psci.c
> @@ -353,6 +353,16 @@ static int kvm_psci_0_1_call(struct kvm_vcpu *vcpu)
>         return 1;
>  }
>
> +static bool kvm_psci_call_is_user(struct kvm_vcpu *vcpu)
> +{
> +       /* Handle the special case of SMCCC probe through PSCI */
> +       if (smccc_get_function(vcpu) == PSCI_1_0_FN_PSCI_FEATURES &&
> +           smccc_get_arg1(vcpu) == ARM_SMCCC_VERSION_FUNC_ID)
> +               return false;
> +
> +       return vcpu->kvm->arch.psci_to_user;
> +}
> +
>  /**
>   * kvm_psci_call - handle PSCI call if r0 value is in range
>   * @vcpu: Pointer to the VCPU struct
> @@ -369,6 +379,9 @@ static int kvm_psci_0_1_call(struct kvm_vcpu *vcpu)
>   */
>  int kvm_psci_call(struct kvm_vcpu *vcpu)
>  {
> +       if (kvm_psci_call_is_user(vcpu))
> +               return kvm_hvc_user(vcpu);
> +
>         switch (kvm_psci_version(vcpu, vcpu->kvm)) {
>         case KVM_ARM_PSCI_1_0:
>                 return kvm_psci_1_0_call(vcpu);
> --
> 2.31.1
>
> _______________________________________________
> kvmarm mailing list
> kvmarm@xxxxxxxxxxxxxxxxxxxxx
> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm