[PATCH v1 07/12] kvm: Add arch-generic interface for hw-accelerated dirty-bitmap cleaning

Leonardo Bras
2026-04-30T11:16:12Z

Introduce kvm_arch_dirty_log_clear() that allow implementation of
arch-specific hardware-accelerated dirty-log routines.

A call to that is added on both kvm_get_dirty_log_protect() and
kvm_clear_dirty_log_protect() and will fall back to software version if
not implemented, or any error was detected in the arch-specific routine.

For an arch to implement this function, it's required to provide an
asm/kvm_dirty_bit.h and have CONFIG_HAVE_KVM_HW_DIRTY_BIT=y on building.

If the arch does not implement it, and thus lack above config, the
introduced snippet is expected to be compiled-out and have zero impact at
runtime.

Signed-off-by: Leonardo Bras <leo.bras@arm.com>
---
include/linux/kvm_dirty_bit.h | 27 +++++++++++++++++++++++++++
virt/kvm/kvm_main.c | 13 ++++++++++++-
virt/kvm/Kconfig | 3 +++
3 files changed, 42 insertions(+), 1 deletion(-)
create mode 100644 include/linux/kvm_dirty_bit.h

diff --git a/include/linux/kvm_dirty_bit.h b/include/linux/kvm_dirty_bit.h
new file mode 100644
index 000000000000..fa4f6b67b623
--- /dev/null
+++ b/include/linux/kvm_dirty_bit.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2026 ARM Ltd.
+ * Author: Leonardo Bras <leo.bras@arm.com>
+ */
+
+#ifndef __KVM_DIRTY_BIT_H__
+#define __KVM_DIRTY_BIT_H__
+
+#ifndef CONFIG_HAVE_KVM_HW_DIRTY_BIT
+
+static inline int kvm_arch_dirty_log_clear(struct kvm *kvm,
+ struct kvm_memory_slot *memslot,
+ struct kvm_clear_dirty_log *log,
+ unsigned long *bitmap,
+ bool *flush)
+{
+ return -ENXIO;
+}
+
+#else /* CONFIG_HAVE_KVM_HW_DIRTY_BIT */
+
+#include <asm/kvm_dirty_bit.h>
+
+#endif /* CONFIG_HAVE_KVM_HW_DIRTY_BIT */
+
+#endif /* __KVM_DIRTY_BIT_H__ */
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 89489996fbc1..7f5048ca9a25 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -58,20 +58,21 @@
#include "async_pf.h"
#include "kvm_mm.h"
#include "vfio.h"

#include <trace/events/ipi.h>

#define CREATE_TRACE_POINTS
#include <trace/events/kvm.h>

#include <linux/kvm_dirty_ring.h>
+#include <linux/kvm_dirty_bit.h>

/* Worst case buffer size needed for holding an integer. */
#define ITOA_MAX_LEN 12

MODULE_AUTHOR("Qumranet");
MODULE_DESCRIPTION("Kernel-based Virtual Machine (KVM) Hypervisor");
MODULE_LICENSE("GPL");

/* Architectures should define their poll value according to the halt latency */
@@ -2255,39 +2256,44 @@ static int kvm_get_dirty_log_protect(struct kvm *kvm, struct kvm_dirty_log *log)
* is some code duplication between this function and
* kvm_get_dirty_log, but hopefully all architecture
* transition to kvm_get_dirty_log_protect and kvm_get_dirty_log
* can be eliminated.
*/
dirty_bitmap_buffer = dirty_bitmap;
} else {
dirty_bitmap_buffer = kvm_second_dirty_bitmap(memslot);
memset(dirty_bitmap_buffer, 0, n);

+ if (kvm_arch_dirty_log_clear(kvm, memslot, NULL,
+ dirty_bitmap_buffer, &flush) >= 0)
+ goto out;
+
KVM_MMU_LOCK(kvm);
for (i = 0; i < n / sizeof(long); i++) {
unsigned long mask;
gfn_t offset;

if (!dirty_bitmap[i])
continue;

flush = true;
mask = xchg(&dirty_bitmap[i], 0);
dirty_bitmap_buffer[i] = mask;

offset = i * BITS_PER_LONG;
kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
offset, mask);
}
KVM_MMU_UNLOCK(kvm);
}

+out:
if (flush)
kvm_flush_remote_tlbs_memslot(kvm, memslot);

if (copy_to_user(log->dirty_bitmap, dirty_bitmap_buffer, n))
return -EFAULT;
return 0;
}

/**
@@ -2366,45 +2372,50 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
(log->num_pages < memslot->npages - log->first_page && (log->num_pages & 63)))
return -EINVAL;

kvm_arch_sync_dirty_log(kvm, memslot);

flush = false;
dirty_bitmap_buffer = kvm_second_dirty_bitmap(memslot);
if (copy_from_user(dirty_bitmap_buffer, log->dirty_bitmap, n))
return -EFAULT;

+ if (kvm_arch_dirty_log_clear(kvm, memslot, log, dirty_bitmap_buffer,
+ &flush) >= 0)
+ goto out;
+
KVM_MMU_LOCK(kvm);
for (offset = log->first_page, i = offset / BITS_PER_LONG,
n = DIV_ROUND_UP(log->num_pages, BITS_PER_LONG); n--;
i++, offset += BITS_PER_LONG) {
unsigned long mask = *dirty_bitmap_buffer++;
atomic_long_t *p = (atomic_long_t *) &dirty_bitmap[i];
if (!mask)
continue;

mask &= atomic_long_fetch_andnot(mask, p);

/*
* mask contains the bits that really have been cleared. This
* never includes any bits beyond the length of the memslot (if
* the length is not aligned to 64 pages), therefore it is not
* a problem if userspace sets them in log->dirty_bitmap.
*/
if (mask) {
flush = true;
+
kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
offset, mask);
}
}
KVM_MMU_UNLOCK(kvm);
-
+out:
if (flush)
kvm_flush_remote_tlbs_memslot(kvm, memslot);

return 0;
}

static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
struct kvm_clear_dirty_log *log)
{
int r;
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 794976b88c6f..f8757b5b84b3 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -13,20 +13,23 @@ config HAVE_KVM_PFNCACHE

config HAVE_KVM_IRQCHIP
bool

config HAVE_KVM_IRQ_ROUTING
bool

config HAVE_KVM_DIRTY_RING
bool

+config HAVE_KVM_HW_DIRTY_BIT
+ 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.
--
2.54.0