Block I/O: Scatter-gather support for virtio requests

Summary

Implement scatter-gather support in the virtio block driver to submit multiple non-contiguous sectors in a single virtio request, improving throughput for fragmented I/O patterns.

Current state

• The virtio block driver (panda-kernel/src/drivers/virtio_block/) supports async I/O with interrupt-driven completion
• Each request currently transfers a single contiguous range of sectors
• Multiple small reads (e.g., from ext2 indirect block lookups) each require a separate virtio request

Desired behaviour

• A single virtio block request can reference multiple non-contiguous (physical) memory buffers
• The virtio descriptor chain uses indirect descriptors or chained descriptors to express scatter-gather lists
• The ext2 driver and VFS layer can submit batched read requests
• Fewer virtio queue submissions for the same amount of data

Key files

• panda-kernel/src/drivers/virtio_block/ — virtio block driver
• panda-kernel/src/drivers/virtio/ — virtio queue infrastructure (descriptor chains)
• panda-kernel/src/vfs/ext2/ — ext2 filesystem driver (primary consumer)

Implementation notes

• Virtio spec supports scatter-gather natively via descriptor chains
• Each descriptor in the chain points to a (physical address, length) pair
• The block device header and status byte are separate descriptors; data descriptors go in between
• Need to handle the case where physical pages are not contiguous even if virtual pages are

Testing

• Kernel test: read multiple non-contiguous sectors in one request, verify data correctness
• Performance test: compare sequential read throughput before and after (measure with OP_ENVIRONMENT_TIME once available)
• Verify ext2 multi-block reads work correctly with scatter-gather

Documentation

• Document the scatter-gather API in the block driver
• Add comments explaining the descriptor chain layout for scatter-gather requests

Start by reading the virtio block driver in panda-kernel/src/drivers/virtio_block/ and the descriptor-chain infrastructure in panda-kernel/src/drivers/virtio/. Then inspect the ext2 consumer in panda-kernel/src/vfs/ext2/ and the existing async request path. Done means non-contiguous buffers can be submitted in one request, ext2 multi-block reads remain correct, and the requested kernel and performance tests are covered.