[PATCH v2 2/6] KVM: Add KVM_CAP_DIRTY_LOG_RING_ACQ_REL capability and config option

[Marc Zyngier <maz@kernel.org>]

In order to differenciate between architectures that require no extra
synchronisation when accessing the dirty ring and those who do,
add a new capability (KVM_CAP_DIRTY_LOG_RING_ACQ_REL) that identify
the latter sort. TSO architectures can obviously advertise both, while
relaxed architectures must only advertise the ACQ_REL version.

This requires some configuration symbol rejigging, with HAVE_KVM_DIRTY_RING
being only indirectly selected by two top-level config symbols:
- HAVE_KVM_DIRTY_RING_TSO for strongly ordered architectures (x86)
- HAVE_KVM_DIRTY_RING_ACQ_REL for weakly ordered architectures (arm64)

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/x86/kvm/Kconfig     |  2 +-
 include/uapi/linux/kvm.h |  1 +
 virt/kvm/Kconfig         | 14 ++++++++++++++
 virt/kvm/kvm_main.c      |  9 ++++++++-
 4 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index e3cbd7706136..876748b236ff 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -28,7 +28,7 @@ config KVM
 	select HAVE_KVM_IRQCHIP
 	select HAVE_KVM_PFNCACHE
 	select HAVE_KVM_IRQFD
-	select HAVE_KVM_DIRTY_RING
+	select HAVE_KVM_DIRTY_RING_TSO
 	select IRQ_BYPASS_MANAGER
 	select HAVE_KVM_IRQ_BYPASS
 	select HAVE_KVM_IRQ_ROUTING
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index eed0315a77a6..0d5d4419139a 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1177,6 +1177,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_VM_DISABLE_NX_HUGE_PAGES 220
 #define KVM_CAP_S390_ZPCI_OP 221
 #define KVM_CAP_S390_CPU_TOPOLOGY 222
+#define KVM_CAP_DIRTY_LOG_RING_ACQ_REL 223
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index a8c5c9f06b3c..800f9470e36b 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -19,6 +19,20 @@ config HAVE_KVM_IRQ_ROUTING
 config HAVE_KVM_DIRTY_RING
        bool
 
+# Only strongly ordered architectures can select this, as it doesn't
+# put any explicit constraint on userspace ordering. They can also
+# select the _ACQ_REL version.
+config HAVE_KVM_DIRTY_RING_TSO
+       bool
+       select HAVE_KVM_DIRTY_RING
+       depends on X86
+
+# Weakly ordered architectures can only select this, advertising
+# to userspace the additional ordering requirements.
+config HAVE_KVM_DIRTY_RING_ACQ_REL
+       bool
+       select HAVE_KVM_DIRTY_RING
+
 config HAVE_KVM_EVENTFD
        bool
        select EVENTFD
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 584a5bab3af3..5b064dbadaf4 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4475,7 +4475,13 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 	case KVM_CAP_NR_MEMSLOTS:
 		return KVM_USER_MEM_SLOTS;
 	case KVM_CAP_DIRTY_LOG_RING:
-#ifdef CONFIG_HAVE_KVM_DIRTY_RING
+#ifdef CONFIG_HAVE_KVM_DIRTY_RING_TSO
+		return KVM_DIRTY_RING_MAX_ENTRIES * sizeof(struct kvm_dirty_gfn);
+#else
+		return 0;
+#endif
+	case KVM_CAP_DIRTY_LOG_RING_ACQ_REL:
+#ifdef CONFIG_HAVE_KVM_DIRTY_RING_ACQ_REL
 		return KVM_DIRTY_RING_MAX_ENTRIES * sizeof(struct kvm_dirty_gfn);
 #else
 		return 0;
@@ -4580,6 +4586,7 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
 		return 0;
 	}
 	case KVM_CAP_DIRTY_LOG_RING:
+	case KVM_CAP_DIRTY_LOG_RING_ACQ_REL:
 		return kvm_vm_ioctl_enable_dirty_log_ring(kvm, cap->args[0]);
 	default:
 		return kvm_vm_ioctl_enable_cap(kvm, cap);
-- 
2.34.1

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