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Date: Thu, 10 Jun 2021 17:02:35 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Jean-Philippe Brucker <jean-philippe@...aro.org>
CC: <maz@...nel.org>, <kvm@...r.kernel.org>,
<kvmarm@...ts.cs.columbia.edu>,
<linux-arm-kernel@...ts.infradead.org>,
<linux-kernel@...r.kernel.org>, <pbonzini@...hat.com>,
<corbet@....net>, <james.morse@....com>,
<alexandru.elisei@....com>, <suzuki.poulose@....com>,
<catalin.marinas@....com>, <will@...nel.org>,
<lorenzo.pieralisi@....com>, <salil.mehta@...wei.com>,
<shameerali.kolothum.thodi@...wei.com>
Subject: Re: [RFC PATCH 4/5] KVM: arm64: Pass hypercalls to userspace
On Tue, 8 Jun 2021 17:48:05 +0200
Jean-Philippe Brucker <jean-philippe@...aro.org> wrote:
> Let userspace request to handle all hypercalls that aren't handled by
> KVM, by setting the KVM_CAP_ARM_HVC_TO_USER capability.
>
> With the help of another capability, this will allow userspace to handle
> PSCI calls.
>
> Suggested-by: James Morse <james.morse@....com>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@...aro.org>
Trivial question inline.
> ---
>
> Notes on this implementation:
>
> * A similar mechanism was proposed for SDEI some time ago [1]. This RFC
> generalizes the idea to all hypercalls, since that was suggested on
> the list [2, 3].
>
> * We're reusing kvm_run.hypercall. I copied x0-x5 into
> kvm_run.hypercall.args[] to help userspace but I'm tempted to remove
> this, because:
> - Most user handlers will need to write results back into the
> registers (x0-x3 for SMCCC), so if we keep this shortcut we should
> go all the way and synchronize them on return to kernel.
> - QEMU doesn't care about this shortcut, it pulls all vcpu regs before
> handling the call.
> - SMCCC uses x0-x16 for parameters.
> x0 does contain the SMCCC function ID and may be useful for fast
> dispatch, we could keep that plus the immediate number.
>
> * Should we add a flag in the kvm_run.hypercall telling whether this is
> HVC or SMC? Can be added later in those bottom longmode and pad
> fields.
>
> * On top of this we could share with userspace which HVC ranges are
> available and which ones are handled by KVM. That can actually be
> added independently, through a vCPU/VM device attribute (which doesn't
> consume a new ioctl):
> - userspace issues HAS_ATTR ioctl on the VM fd to query whether this
> feature is available.
> - userspace queries the number N of HVC ranges using one GET_ATTR.
> - userspace passes an array of N ranges using another GET_ATTR.
> The array is filled and returned by KVM.
>
> * Untested for AArch32 guests.
>
> [1] https://lore.kernel.org/linux-arm-kernel/20170808164616.25949-12-james.morse@arm.com/
> [2] https://lore.kernel.org/linux-arm-kernel/bf7e83f1-c58e-8d65-edd0-d08f27b8b766@arm.com/
> [3] https://lore.kernel.org/linux-arm-kernel/f56cf420-affc-35f0-2355-801a924b8a35@arm.com/
> ---
> Documentation/virt/kvm/api.rst | 17 +++++++++++++++--
> arch/arm64/include/asm/kvm_host.h | 1 +
> include/kvm/arm_psci.h | 4 ++++
> include/uapi/linux/kvm.h | 1 +
> arch/arm64/kvm/arm.c | 5 +++++
> arch/arm64/kvm/hypercalls.c | 28 +++++++++++++++++++++++++++-
> 6 files changed, 53 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index e4fe7fb60d5d..3d8c1661e7b2 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -5228,8 +5228,12 @@ to the byte array.
> __u32 pad;
> } 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).
> +On x86 this was once used for 'hypercall to userspace'. To implement such
> +functionality, use KVM_EXIT_IO (x86) or KVM_EXIT_MMIO (all except s390).
> +
> +On arm64 it is used for hypercalls, when the KVM_CAP_ARM_HVC_TO_USER capability
> +is enabled. 'nr' contains the HVC or SMC immediate. 'args' contains registers
> +x0 - x5. The other parameters are unused.
>
> .. note:: KVM_EXIT_IO is significantly faster than KVM_EXIT_MMIO.
>
> @@ -6894,3 +6898,12 @@ This capability is always enabled.
> This capability indicates that the KVM virtual PTP service is
> supported in the host. A VMM can check whether the service is
> available to the guest on migration.
> +
> +8.33 KVM_CAP_ARM_HVC_TO_USER
> +----------------------------
> +
> +:Architecture: arm64
> +
> +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.
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 3ca732feb9a5..25554ce97045 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -123,6 +123,7 @@ struct kvm_arch {
> * supported.
> */
> bool return_nisv_io_abort_to_user;
> + bool hvc_to_user;
>
> /*
> * VM-wide PMU filter, implemented as a bitmap and big enough for
> diff --git a/include/kvm/arm_psci.h b/include/kvm/arm_psci.h
> index 5b58bd2fe088..d6b71a48fbb1 100644
> --- a/include/kvm/arm_psci.h
> +++ b/include/kvm/arm_psci.h
> @@ -16,6 +16,10 @@
>
> #define KVM_ARM_PSCI_LATEST KVM_ARM_PSCI_1_0
>
> +#define KVM_PSCI_FN_LAST KVM_PSCI_FN(3)
> +#define PSCI_0_2_FN_LAST PSCI_0_2_FN(0x3f)
> +#define PSCI_0_2_FN64_LAST PSCI_0_2_FN64(0x3f)
Why 3f? Perhaps a spec reference if appropriate.
> +
> /*
> * We need the KVM pointer independently from the vcpu as we can call
> * this from HYP, and need to apply kern_hyp_va on it...
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 06ba64c49737..aa831986a399 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1084,6 +1084,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_VM_COPY_ENC_CONTEXT_FROM 197
> #define KVM_CAP_PTP_KVM 198
> #define KVM_CAP_ARM_MP_HALTED 199
> +#define KVM_CAP_ARM_HVC_TO_USER 200
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index d6ad977fea5f..074197721e97 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -93,6 +93,10 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> r = 0;
> kvm->arch.return_nisv_io_abort_to_user = true;
> break;
> + case KVM_CAP_ARM_HVC_TO_USER:
> + r = 0;
> + kvm->arch.hvc_to_user = true;
> + break;
> default:
> r = -EINVAL;
> break;
> @@ -208,6 +212,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_VCPU_ATTRIBUTES:
> case KVM_CAP_PTP_KVM:
> case KVM_CAP_ARM_MP_HALTED:
> + case KVM_CAP_ARM_HVC_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 30da78f72b3b..ccc2015eddf9 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -58,6 +58,28 @@ 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 i;
> + struct kvm_run *run = vcpu->run;
> +
> + if (!vcpu->kvm->arch.hvc_to_user) {
> + smccc_set_retval(vcpu, SMCCC_RET_NOT_SUPPORTED, 0, 0, 0);
> + return 1;
> + }
> +
> + run->exit_reason = KVM_EXIT_HYPERCALL;
> + run->hypercall.nr = kvm_vcpu_hvc_get_imm(vcpu);
> + /* Copy the first parameters for fast access */
> + for (i = 0; i < 6; i++)
> + run->hypercall.args[i] = vcpu_get_reg(vcpu, i);
> + run->hypercall.ret = 0;
> + run->hypercall.longmode = 0;
> + run->hypercall.pad = 0;
> +
> + return 0;
> +}
> +
> int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
> {
> u32 func_id = smccc_get_function(vcpu);
> @@ -139,8 +161,12 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
> case ARM_SMCCC_TRNG_RND32:
> case ARM_SMCCC_TRNG_RND64:
> return kvm_trng_call(vcpu);
> - default:
> + case KVM_PSCI_FN_BASE...KVM_PSCI_FN_LAST:
> + case PSCI_0_2_FN_BASE...PSCI_0_2_FN_LAST:
> + case PSCI_0_2_FN64_BASE...PSCI_0_2_FN64_LAST:
> return kvm_psci_call(vcpu);
> + default:
> + return kvm_hvc_user(vcpu);
> }
>
> smccc_set_retval(vcpu, val[0], val[1], val[2], val[3]);
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