[PATCH 4/5] KVM: selftests: Reserve memslot 1 for the ucall MMIO address

From: Tharit Tangkijwanichakul <tharitt97@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>
Cc: Shuah Khan <shuah@kernel.org>, Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oupton@kernel.org>,
	Fuad Tabba <fuad.tabba@linux.dev>,
	Joey Gouly <joey.gouly@arm.com>,
	Steffen Eiden <seiden@linux.ibm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Anup Patel <anup@brainfault.org>,
	Atish Patra <atish.patra@linux.dev>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
	linux-kernel-mentees@lists.linux.dev, skhan@linuxfoundation.org,
	me@brighamcampbell.com, jkoolstra@xs4all.nl,
	Tharit Tangkijwanichakul <tharitt97@gmail.com>
Subject: [PATCH 4/5] KVM: selftests: Reserve memslot 1 for the ucall MMIO address
Date: Sat, 17 Oct 2026 18:02:12 +0700	[thread overview]
Message-ID: <20261017110213.46199-5-tharitt97@gmail.com> (raw)
In-Reply-To: <20261017110213.46199-1-tharitt97@gmail.com>

Back the ucall MMIO address with a read-only region in slot 1
when KVM_CAP_READONLY_MEM is available. Writes still exit as
MMIO, while overlapping memslot creation is rejected.

Start test memslots at slot 2, leaving slot 1 reserved by
convention even when read-only memory is unsupported.

Signed-off-by: Tharit Tangkijwanichakul <tharitt97@gmail.com>
---
 .../testing/selftests/kvm/include/kvm_util.h  |  2 +-
 tools/testing/selftests/kvm/lib/kvm_util.c    | 19 +++++++++++++------
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/kvm/include/kvm_util.h b/tools/testing/selftests/kvm/include/kvm_util.h
index 759d604d44ce..4dbb2ef332fc 100644
--- a/tools/testing/selftests/kvm/include/kvm_util.h
+++ b/tools/testing/selftests/kvm/include/kvm_util.h
@@ -30,7 +30,7 @@
 
 #define KVM_DEV_PATH "/dev/kvm"
 #define KVM_MAX_VCPUS 512
-#define TEST_MEM_SLOT_BASE 1
+#define TEST_MEM_SLOT_BASE 2
 
 #define NSEC_PER_SEC 1000000000L
 
diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index 9ddc047d5c27..73e28814a226 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -20,6 +20,7 @@
 #include <linux/kernel.h>
 
 #define KVM_UTIL_MIN_PFN	2
+#define KVM_UCALL_MEMSLOT	1
 
 u32 kvm_random_seed;
 struct kvm_random_state kvm_rng;
@@ -492,6 +493,7 @@ struct kvm_vm *__vm_create(struct vm_shape shape, u32 nr_runnable_vcpus,
 	struct userspace_mem_region *slot0;
 	struct kvm_vm *vm;
 	int i, flags;
+	gpa_t mmio_gpa;
 
 	kvm_set_files_rlimit(nr_runnable_vcpus);
 
@@ -514,14 +516,19 @@ struct kvm_vm *__vm_create(struct vm_shape shape, u32 nr_runnable_vcpus,
 
 	kvm_vm_elf_load(vm, program_invocation_name);
 
-	/*
-	 * TODO: Add proper defines to protect the library's memslots, and then
-	 * carve out memslot1 for the ucall MMIO address.  KVM treats writes to
-	 * read-only memslots as MMIO, and creating a read-only memslot for the
-	 * MMIO region would prevent silently clobbering the MMIO region.
+	/* KVM treats writes to read-only memslots as MMIO, and creating
+	 * a read-only memslot for the MMIO region would prevent silently
+	 * clobbering the MMIO region.
 	 */
 	slot0 = memslot2region(vm, 0);
-	ucall_init(vm, slot0->region.guest_phys_addr + slot0->region.memory_size);
+	mmio_gpa = slot0->region.guest_phys_addr + slot0->region.memory_size;
+	ucall_init(vm, mmio_gpa);
+
+	if (vm_check_cap(vm, KVM_CAP_READONLY_MEM))
+		vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
+					    mmio_gpa, KVM_UCALL_MEMSLOT,
+					    vm_adjust_num_guest_pages(vm->mode, 1),
+					    KVM_MEM_READONLY);
 
 	if (kvm_random_seed != last_kvm_seed)
 		kvm_seed_rng(kvm_random_seed);
-- 
2.53.0

