use the configured OS page size in get_dirty_log()

Right now get_dirty_log() acts on the assumption that the page size is
always 4K. This is not necessarily true, since the page size is
configurable. This commit improves the logic by calling sysconf() to
get the actual page size.

Signed-off-by: Serban Iorga <[email protected]>

Author: Serban Iorga

coverage_config.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 92.0,
+  "coverage_score": 92.1,
   "exclude_path": "",
   "crate_features": ""
 }

src/ioctls/vm.rs

@@ -791,22 +791,24 @@ impl VmFd {
     ///
     pub fn get_dirty_log(&self, slot: u32, memory_size: usize) -> Result<Vec<u64>> {
         // Compute the length of the bitmap needed for all dirty pages in one memory slot.
-        // One memory page is 4KiB (4096 bits) and `KVM_GET_DIRTY_LOG` returns one dirty bit for
+        // One memory page is `page_size` bytes and `KVM_GET_DIRTY_LOG` returns one dirty bit for
         // each page.
-        let page_size = 4 << 10;
+        let page_size = match unsafe { libc::sysconf(libc::_SC_PAGESIZE) } {
+            -1 => return Err(errno::Error::last()),
+            ps => ps as usize,
+        };
 
-        let div_round_up = |dividend, divisor| (dividend + divisor - 1) / divisor;
         // For ease of access we are saving the bitmap in a u64 vector. We are using ceil to
-        // make sure we count all dirty pages even when `mem_size` is not a multiple of
-        // page_size * 64.
-        let bitmap_size = div_round_up(memory_size, page_size * 64);
-        let mut bitmap = vec![0; bitmap_size];
-        let b_data = bitmap.as_mut_ptr() as *mut c_void;
+        // make sure we count all dirty pages even when `memory_size` is not a multiple of
+        // `page_size * 64`.
+        let div_ceil = |dividend, divisor| (dividend + divisor - 1) / divisor;
+        let bitmap_size = div_ceil(memory_size, page_size * 64);
+        let mut bitmap = vec![0u64; bitmap_size];
         let dirtylog = kvm_dirty_log {
             slot,
             padding1: 0,
             __bindgen_anon_1: kvm_dirty_log__bindgen_ty_1 {
-                dirty_bitmap: b_data,
+                dirty_bitmap: bitmap.as_mut_ptr() as *mut c_void,
             },
         };
         // Safe because we know that our file is a VM fd, and we know that the amount of memory