On 2025/10/21 16:25, Duan, Zhenzhong wrote:
-----Original Message-----
From: Liu, Yi L <[email protected]>
Subject: Re: [PATCH v2 7/8] vfio/migration: Add migration blocker if VM
memory is too large to cause unmap_bitmap failure
On 2025/10/17 16:22, Zhenzhong Duan wrote:
With default config, kernel VFIO type1 driver limits dirty bitmap to 256MB
for unmap_bitmap ioctl so the maximum guest memory region is no more
than
8TB size for the ioctl to succeed.
Be conservative here to limit total guest memory to 8TB or else add a
migration blocker. IOMMUFD backend doesn't have such limit, one can use
IOMMUFD backed device if there is a need to migration such large VM.
Suggested-by: Yi Liu <[email protected]>
Signed-off-by: Zhenzhong Duan <[email protected]>
---
hw/vfio/migration.c | 37 +++++++++++++++++++++++++++++++++++++
1 file changed, 37 insertions(+)
diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
index 4c06e3db93..1106ca7857 100644
--- a/hw/vfio/migration.c
+++ b/hw/vfio/migration.c
@@ -16,6 +16,7 @@
#include <sys/ioctl.h>
#include "system/runstate.h"
+#include "hw/boards.h"
#include "hw/vfio/vfio-device.h"
#include "hw/vfio/vfio-migration.h"
#include "migration/misc.h"
@@ -1152,6 +1153,35 @@ static bool vfio_viommu_preset(VFIODevice
*vbasedev)
return vbasedev->bcontainer->space->as !=
&address_space_memory;
}
+static bool vfio_dirty_tracking_exceed_limit(VFIODevice *vbasedev)
+{
+ VFIOContainer *bcontainer = vbasedev->bcontainer;
+ uint64_t max_size, page_size;
+
+ if (!object_dynamic_cast(OBJECT(bcontainer),
TYPE_VFIO_IOMMU_LEGACY)) {
+ return false;
+ }
+
+ if (!bcontainer->dirty_pages_supported) {
+ return true;
+ }
+ /*
+ * VFIO type1 driver has a limitation of bitmap size on unmap_bitmap
+ * ioctl(), calculate the limit and compare with guest memory size to
+ * catch dirty tracking failure early.
+ *
+ * This limit is 8TB with default kernel and QEMU config, we are a bit
+ * conservative here as VM memory layout may be nonconsecutive
or VM
+ * can run with vIOMMU enabled so the limitation could be relaxed.
One
+ * can also switch to use IOMMUFD backend if there is a need to
migrate
+ * large VM.
+ */
+ page_size = 1 << ctz64(bcontainer->dirty_pgsizes);
Should use qemu_real_host_page_size() here?
hmm, I think it's host mmu page size which is not as accurate as the iommu page
sizes? here we want the iommu ones.
I saw vfio_legacy_query_dirty_bitmap() uses qemu_real_host_page_size()
though kernel enforces min iommu page size. Shall we let
qemu_real_host_page_size() use iommu page size instead of cpu page
size?
