block: only advertise VIRTIO_BLK_F_DISCARD when sparse=true - #7936
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March 31, 2026 12:32
PR cloud-hypervisor#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 cloud-hypervisor#7849 Signed-off-by: CMGS <ilskdw@gmail.com>
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March 31, 2026 13:41
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PR #7852 fixed the missing VirtioBlockConfig fields for DISCARD but did not change the feature advertisement logic. The condition sparse || disk_image.supports_zero_flag() causes qcow2 images to advertise DISCARD even with sparse=false, because qcow2 can mark clusters as zero without deallocating (supports_zero_flag() returns true).
Windows viostor driver BSODs with DRIVER_IRQL_NOT_LESS_OR_EQUAL when DISCARD is advertised on qcow2 backends. This makes sparse=off ineffective as a workaround for qcow2 images.
Restrict DISCARD advertisement to explicit sparse=true only. WRITE_ZEROES remains available for all sparse-capable backends.
Fixes: #7849