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block: Restrict DISCARD to explicit sparse=true

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PR #7852 fixed the missing VirtioBlockConfig fields but did not
change the feature advertisement logic. The condition
`sparse || disk_image.supports_zero_flag()` causes qcow2 to
advertise DISCARD even with sparse=false, because qcow2 can
mark clusters as zero (supports_zero_flag() returns true).

Windows viostor BSODs (DRIVER_IRQL_NOT_LESS_OR_EQUAL) when
DISCARD is advertised on qcow2 backends, making sparse=off
ineffective as a workaround for qcow2 images.

Restrict DISCARD to explicit sparse=true only. WRITE_ZEROES
remains available for all sparse-capable backends.

Fixes #7849

Signed-off-by: CMGS <ilskdw@gmail.com>
This commit is contained in:
CMGS
2026-03-31 08:50:04 +00:00
committed by Rob Bradford
parent 8ca5210603
commit e4e2a37afa
2 changed files with 35 additions and 35 deletions
Download Patch File Download Diff File Expand all files Collapse all files

61
cloud-hypervisor/tests/integration.rs
View File

@@ -4564,7 +4564,6 @@ mod common_parallel {
#[test]
fn test_virtio_block_sparse_off_qcow2() {
const TEST_DISK_SIZE: &str = "2G";
const CLUSTER_SIZE_BYTES: u64 = 64 * 1024;
let disk_config = UbuntuDiskConfig::new(JAMMY_IMAGE_NAME.to_string());
let guest = Guest::new(Box::new(disk_config));
@@ -4618,37 +4617,36 @@ mod common_parallel {
1
);
let mut current_offset_kb = 1024;
for &size_kb in BLOCK_DISCARD_TEST_SIZES_KB.iter() {
guest
.ssh_command(&format!(
"sudo dd if=/dev/urandom of=/dev/vdc bs=1K count={size_kb} seek={current_offset_kb} oflag=direct"
))
.unwrap();
guest.ssh_command("sync").unwrap();
guest
.ssh_command(&format!(
"sudo blkdiscard -o {} -l {} /dev/vdc",
current_offset_kb * 1024,
size_kb * 1024
))
.unwrap();
guest.ssh_command("sync").unwrap();
// Verify VM sees zeros in discarded region
assert_guest_disk_region_is_zero(
&guest,
"/dev/vdc",
current_offset_kb * 1024,
size_kb * 1024,
// With sparse=off, DISCARD should NOT be advertised.
// blkdiscard is expected to fail.
let discard_result =
guest.ssh_command("sudo blkdiscard -o 1048576 -l 1048576 /dev/vdc 2>&1; echo $?");
let exit_code = discard_result
.unwrap()
.trim()
.lines()
.last()
.unwrap_or("1")
.parse::<u32>()
.unwrap_or(1);
assert_ne!(
exit_code, 0,
"blkdiscard should fail with sparse=off (DISCARD not advertised)"
);
current_offset_kb += size_kb + 64;
}
// WRITE_ZEROES should still work via blkdiscard --zeroout
guest
.ssh_command(
"sudo dd if=/dev/urandom of=/dev/vdc bs=1K count=64 seek=1024 oflag=direct",
)
.unwrap();
guest.ssh_command("sync").unwrap();
guest
.ssh_command("sudo blkdiscard -z -o 1048576 -l 65536 /dev/vdc")
.unwrap();
guest.ssh_command("sync").unwrap();
assert_guest_disk_region_is_zero(&guest, "/dev/vdc", 1048576, 65536);
});
kill_child(&mut child);
@@ -4659,9 +4657,10 @@ mod common_parallel {
handle_child_output(r, &output);
// WRITE_ZEROES should still produce zero-flagged regions
assert!(
zero_regions_after > zero_regions_before,
"Expected zero-flagged regions to increase with sparse=off: before={zero_regions_before}, after={zero_regions_after}"
"Expected zero-flagged regions to increase via WRITE_ZEROES: before={zero_regions_before}, after={zero_regions_after}"
);
disk_check_consistency(test_disk_path, None);

9
virtio-devices/src/block.rs
View File

@@ -778,13 +778,14 @@ impl Block {
| (1u64 << VIRTIO_RING_F_INDIRECT_DESC);
// When backend supports sparse operations:
// - Always advertise WRITE_ZEROES
// - Advertise DISCARD only if sparse=true OR format supports marking
// clusters as zero without deallocating
// - Always advertise WRITE_ZEROES (safe for all drivers)
// - Advertise DISCARD only when sparse=true, since DISCARD
// deallocates space via punch_hole and should require
// explicit user opt in.
let mut discard_supported = false;
if disk_image.supports_sparse_operations() {
avail_features |= 1u64 << VIRTIO_BLK_F_WRITE_ZEROES;
if sparse || disk_image.supports_zero_flag() {
if sparse {
avail_features |= 1u64 << VIRTIO_BLK_F_DISCARD;
discard_supported = true;
}