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From:   Jean-Philippe Brucker <jean-philippe@linaro.org>
To:     maz@kernel.org
Cc:     kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        pbonzini@redhat.com, corbet@lwn.net, james.morse@arm.com,
        alexandru.elisei@arm.com, suzuki.poulose@arm.com,
        catalin.marinas@arm.com, will@kernel.org,
        lorenzo.pieralisi@arm.com, salil.mehta@huawei.com,
        shameerali.kolothum.thodi@huawei.com, jonathan.cameron@huawei.com,
        Jean-Philippe Brucker <jean-philippe@linaro.org>
Subject: [RFC PATCH 4/5] KVM: arm64: Pass hypercalls to userspace
Date:   Tue,  8 Jun 2021 17:48:05 +0200
Message-Id: <20210608154805.216869-5-jean-philippe@linaro.org>
In-Reply-To: <20210608154805.216869-1-jean-philippe@linaro.org>
References: <20210608154805.216869-1-jean-philippe@linaro.org>
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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@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
---

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)
+
 /*
  * 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]);
-- 
2.31.1