On Mon, 15 May 2023 10:20:41 -0700, Michael Kelley wrote:
In a SCSI request, storvsc pre-allocates space for up to MAX_PAGE_BUFFER_COUNT physical frame numbers to be passed to Hyper-V. If the size of the I/O request requires more PFNs, a separate memory area of exactly the correct size is dynamically allocated.
But when the pre-allocated area is used, current code always passes MAX_PAGE_BUFFER_COUNT PFNs to Hyper-V, even if fewer are needed. While this doesn't break anything because the additional PFNs are always zero, more bytes than necessary are copied into the VMBus channel ring buffer. This takes CPU cycles and wastes space in the ring buffer. For a typical 4 Kbyte I/O that requires only a single PFN, 248 unnecessary bytes are copied.
[...]
Applied to 6.4/scsi-fixes, thanks!
[1/1] scsi: storvsc: Don't pass unused PFNs to Hyper-V host https://git.kernel.org/mkp/scsi/c/4e81a6cba517