Send page size information to UFFD handler during handshake

When using a page-size agnostic UFFD handler, orchestration becomes
easier if Firecracker tells the UFFD handler about page size (instead of
some orchestrator having to tell both Firecracker and the UFFD handler
about the page size independently). Thus, send along the page size in
the initial payload that Firecracker sends to the UFFD handler (e.g. the
one also containing information about the memory regions).

We send the page size information for each region (even though
Firecracker does not support per-region configuration of huge pages)
for backward compatibility reasons (changing the top-level json object
from a list of region to a dictionary would be a breaking change, but
adding a field to each list entry is not).

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/vmm/src/persist.rs

@@ -102,6 +102,8 @@ pub struct GuestRegionUffdMapping {
     pub size: usize,
     /// Offset in the backend file/buffer where the region contents are.
     pub offset: u64,
+    /// The configured page size for this memory region.
+    pub page_size_kib: usize,
 }
 
 /// Errors related to saving and restoring Microvm state.
@@ -542,6 +544,7 @@ fn guest_memory_from_uffd(
             base_host_virt_addr: host_base_addr as u64,
             size,
             offset: state_region.offset,
+            page_size_kib: huge_pages.page_size_kib(),
         });
     }
 

src/vmm/src/vmm_config/machine_config.rs

@@ -84,6 +84,14 @@ impl HugePageConfig {
     pub fn is_hugetlbfs(&self) -> bool {
         matches!(self, HugePageConfig::Hugetlbfs2M)
     }
+
+    /// Gets the page size in KiB of this [`HugePageConfig`].
+    pub fn page_size_kib(&self) -> usize {
+        match self {
+            HugePageConfig::None => 4096,
+            HugePageConfig::Hugetlbfs2M => 2 * 1024 * 1024,
+        }
+    }
 }
 
 impl From<HugePageConfig> for Option<memfd::HugetlbSize> {