On 3/20/24 09:39, Michael Roth wrote:
Some subsystems like VFIO might disable ram block discard for
uncoordinated cases. Since kvm_convert_memory()/guest_memfd don't
implement a RamDiscardManager handler to convey discard operations to
various listeners like VFIO. > Because of this, sequences like the
following can result due to stale IOMMU mappings:
Alternatively, should guest-memfd memory regions call
ram_block_discard_require(true)? This will prevent VFIO from operating,
but it will avoid consuming twice the memory.
If desirable, guest-memfd support can be changed to implement an
extension of RamDiscardManager that notifies about private/shared memory
changes, and then guest-memfd would be able to support coordinated
discard. But I wonder if that's doable at all - how common are
shared<->private flips, and is it feasible to change the IOMMU page
tables every time?
If the real solution is SEV-TIO (which means essentially guest_memfd
support for VFIO), calling ram_block_discard_require(true) may be the
simplest stopgap solution.
Paolo
- convert page shared->private
- discard shared page
- convert page private->shared
- new page is allocated
- issue DMA operations against that shared page
Address this by taking ram_block_discard_is_enabled() into account when
deciding whether or not to discard pages.
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
---
accel/kvm/kvm-all.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 53ce4f091e..6ae03c880f 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -2962,10 +2962,14 @@ static int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private)
*/
return 0;
} else {
- ret = ram_block_discard_range(rb, offset, size);
+ ret = ram_block_discard_is_disabled()
+ ? ram_block_discard_range(rb, offset, size)
+ : 0;
}
} else {
- ret = ram_block_discard_guest_memfd_range(rb, offset, size);
+ ret = ram_block_discard_is_disabled()
+ ? ram_block_discard_guest_memfd_range(rb, offset, size)
+ : 0;
}
} else {
error_report("Convert non guest_memfd backed memory region "
