[PATCH 4/5] drivers/virt: pkvm: Handle a granule larger than PAGE_SIZE for MMIO_GUARD

[Vincent Donnefort]

In preparation for allowing protected VMs to run on a system where the
granule is bigger than their PAGE_SIZE, allow MMIO_GUARD requests to
overshoot.

Validate the memory regions are aligned with the hypervisor granule
before enabling the MMIO_GUARD support. If aligned, then the MMIO
regions are and overshooting is safe as MMIO_GUARD is solely here to
indicate to the hypervisor where the MMIO regions are.

It is possible to add new memory regions with memory hotplug later.
However the alignment requirement is way more conservative than the
maximum granule size of 64K. Nonetheless, add a test to document the
limitation.

Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
---
 drivers/virt/coco/pkvm-guest/arm-pkvm-guest.c | 51 ++++++++++++++++---
 1 file changed, 43 insertions(+), 8 deletions(-)

diff --git a/drivers/virt/coco/pkvm-guest/arm-pkvm-guest.c b/drivers/virt/coco/pkvm-guest/arm-pkvm-guest.c
index 98b1026cf68b..3852be6bd16b 100644
--- a/drivers/virt/coco/pkvm-guest/arm-pkvm-guest.c
+++ b/drivers/virt/coco/pkvm-guest/arm-pkvm-guest.c
@@ -12,6 +12,8 @@
 #include <linux/init.h>
 #include <linux/io.h>
 #include <linux/mem_encrypt.h>
+#include <linux/memblock.h>
+#include <linux/memory.h>
 #include <linux/mm.h>
 #include <linux/pgtable.h>
 
@@ -70,17 +72,50 @@ static int mmio_guard_ioremap_hook(phys_addr_t phys, size_t size,
 	if (protval != PROT_DEVICE_nGnRE && protval != PROT_DEVICE_nGnRnE)
 		return 0;
 
-	end = PAGE_ALIGN(phys + size);
-	phys = PAGE_ALIGN_DOWN(phys);
+	/*
+	 * It is fine to overshoot MMIO_GUARD requests. Its sole purpose is to
+	 * indicate to the hypervisor where the MMIO regions are and we have
+	 * validated the alignment of the memory regions beforehand.
+	 */
+	end = ALIGN(phys + size, max(pkvm_granule, PAGE_SIZE));
+	phys = ALIGN_DOWN(phys, max(pkvm_granule, PAGE_SIZE));
 
-	while (phys < end) {
-		const int func_id = ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_FUNC_ID;
+	WARN_ON_ONCE(arm_smccc_do_range(ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_FUNC_ID,
+					phys, end - phys));
+	return 0;
+}
 
-		WARN_ON_ONCE(arm_smccc_do_range(func_id, phys, PAGE_SIZE));
-		phys += PAGE_SIZE;
+/*
+ * Return true if the MMIO_GUARD service is available and if overshooting is
+ * safe, which it is if the memory regions are aligned with pkvm_granule.
+ */
+static bool __init mmio_guard_available(void)
+{
+	struct memblock_region *region;
+	phys_addr_t prev_end = 0;
+
+	if (!kvm_arm_hyp_service_available(ARM_SMCCC_KVM_FUNC_MMIO_GUARD))
+		return false;
+
+	if (pkvm_granule <= PAGE_SIZE)
+		return true;
+
+	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG) &&
+	    pkvm_granule > memory_block_size_bytes())
+		return false;
+
+	for_each_mem_region(region) {
+		if (prev_end == region->base)
+			goto contiguous;
+
+		if (!IS_ALIGNED(prev_end | region->base, pkvm_granule))
+			return false;
+
+contiguous:
+		prev_end = region->base + region->size;
 	}
 
-	return 0;
+	return IS_ALIGNED(prev_end, pkvm_granule);
 }
 
 void __init pkvm_init_hyp_services(void)
@@ -108,7 +143,7 @@ void __init pkvm_init_hyp_services(void)
 		pr_info("pKVM: sharing memory in %zu-byte granules\n", pkvm_granule);
 	arm64_mem_crypt_ops_register(&pkvm_crypt_ops);
 
-	if (kvm_arm_hyp_service_available(ARM_SMCCC_KVM_FUNC_MMIO_GUARD))
+	if (mmio_guard_available())
 		arm64_ioremap_prot_hook_register(&mmio_guard_ioremap_hook);
 
 	static_branch_enable(&pkvm_guest);
-- 
2.56.0.rc1.315.gc6ed9934b7-goog