Re: [PATCH v3 19/49] kvm: Make kvm_convert_memory() obey ram_block_discard_is_enabled()

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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 "





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