drivers/virt: pkvm: Intercept ioremap using pKVM MMIO_GUARD hypercall

Hook up pKVM's MMIO_GUARD hypercall so that ioremap() and friends will
register the target physical address as MMIO with the hypervisor,
allowing guest exits to that page to be emulated by the host with full
syndrome information.

Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240830130150.8568-7-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

Documentation/virt/kvm/arm/hypercalls.rst

@@ -116,3 +116,29 @@ memory protection granule advertised by ``ARM_SMCCC_KVM_FUNC_HYP_MEMINFO``.
 |                     |          |    +---------------------------------------------+
 |                     |          |    | ``INVALID_PARAMETER (-3)``                  |
 +---------------------+----------+----+---------------------------------------------+
+
+``ARM_SMCCC_KVM_FUNC_MMIO_GUARD``
+----------------------------------
+
+Request that a given memory region is handled as MMIO by the hypervisor,
+allowing accesses to this region to be emulated by the KVM host. The size of the
+region is equal to the memory protection granule advertised by
+``ARM_SMCCC_KVM_FUNC_HYP_MEMINFO``.
+
++---------------------+-------------------------------------------------------------+
+| Presence:           | Optional; pKVM protected guests only.                       |
++---------------------+-------------------------------------------------------------+
+| Calling convention: | HVC64                                                       |
++---------------------+----------+--------------------------------------------------+
+| Function ID:        | (uint32) | 0xC6000007                                       |
++---------------------+----------+----+---------------------------------------------+
+| Arguments:          | (uint64) | R1 | Base IPA of MMIO memory region              |
+|                     +----------+----+---------------------------------------------+
+|                     | (uint64) | R2 | Reserved / Must be zero                     |
+|                     +----------+----+---------------------------------------------+
+|                     | (uint64) | R3 | Reserved / Must be zero                     |
++---------------------+----------+----+---------------------------------------------+
+| Return Values:      | (int64)  | R0 | ``SUCCESS (0)``                             |
+|                     |          |    +---------------------------------------------+
+|                     |          |    | ``INVALID_PARAMETER (-3)``                  |
++---------------------+----------+----+---------------------------------------------+

drivers/virt/coco/pkvm-guest/arm-pkvm-guest.c

@@ -9,8 +9,10 @@
 
 #include <linux/arm-smccc.h>
 #include <linux/array_size.h>
+#include <linux/io.h>
 #include <linux/mem_encrypt.h>
 #include <linux/mm.h>
+#include <linux/pgtable.h>
 
 #include <asm/hypervisor.h>
 
@@ -67,6 +69,36 @@ static const struct arm64_mem_crypt_ops pkvm_crypt_ops = {
 	.decrypt	= pkvm_set_memory_decrypted,
 };
 
+static int mmio_guard_ioremap_hook(phys_addr_t phys, size_t size,
+				   pgprot_t *prot)
+{
+	phys_addr_t end;
+	pteval_t protval = pgprot_val(*prot);
+
+	/*
+	 * We only expect MMIO emulation for regions mapped with device
+	 * attributes.
+	 */
+	if (protval != PROT_DEVICE_nGnRE && protval != PROT_DEVICE_nGnRnE)
+		return 0;
+
+	phys = PAGE_ALIGN_DOWN(phys);
+	end = phys + PAGE_ALIGN(size);
+
+	while (phys < end) {
+		const int func_id = ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_FUNC_ID;
+		int err;
+
+		err = arm_smccc_do_one_page(func_id, phys);
+		if (err)
+			return err;
+
+		phys += PAGE_SIZE;
+	}
+
+	return 0;
+}
+
 void pkvm_init_hyp_services(void)
 {
 	int i;
@@ -89,4 +121,7 @@ void pkvm_init_hyp_services(void)
 
 	pkvm_granule = res.a0;
 	arm64_mem_crypt_ops_register(&pkvm_crypt_ops);
+
+	if (kvm_arm_hyp_service_available(ARM_SMCCC_KVM_FUNC_MMIO_GUARD))
+		arm64_ioremap_prot_hook_register(&mmio_guard_ioremap_hook);
 }

include/linux/arm-smccc.h

@@ -118,6 +118,7 @@
 #define ARM_SMCCC_KVM_FUNC_HYP_MEMINFO		2
 #define ARM_SMCCC_KVM_FUNC_MEM_SHARE		3
 #define ARM_SMCCC_KVM_FUNC_MEM_UNSHARE		4
+#define ARM_SMCCC_KVM_FUNC_MMIO_GUARD		7
 #define ARM_SMCCC_KVM_FUNC_FEATURES_2		127
 #define ARM_SMCCC_KVM_NUM_FUNCS			128
 
@@ -158,6 +159,12 @@
 			   ARM_SMCCC_OWNER_VENDOR_HYP,			\
 			   ARM_SMCCC_KVM_FUNC_MEM_UNSHARE)
 
+#define ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_FUNC_ID			\
+	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
+			   ARM_SMCCC_SMC_64,				\
+			   ARM_SMCCC_OWNER_VENDOR_HYP,			\
+			   ARM_SMCCC_KVM_FUNC_MMIO_GUARD)
+
 /* ptp_kvm counter type ID */
 #define KVM_PTP_VIRT_COUNTER			0
 #define KVM_PTP_PHYS_COUNTER			1