Laszlo Ersek 8bc951a264 OvmfPkg: VirtioFlush(): return the number of bytes written by the host ... VirtioLib provides an API for simple, synchronous (request/response-style) virtio communication. The guest driver builds one descriptor chain, link for link, with VirtioPrepare() and VirtioAppendDesc(), then submits the chain, and awaits the processing, with VirtioFlush(). The descriptor chain is always built at the beginning of the descriptor area, with the head descriptor having descriptor index 0. In order to submit the descriptor chain to the host, the guest always pushes a new "available element" to the Available Ring, in genuine queue-like fashion, with the new element referencing the head descriptor (which always has index 0, see above). In turn, after processing, the host always pushes a new "used element" to the Used Ring, in genuine queue-like fashion, with the new element referencing the head descriptor of the chain that was just processed. The same element also reports the number of bytes that the host wrote, consecutively across the host-writeable buffers that were linked by the descriptors. (See "OvmfPkg/VirtioNetDxe/TechNotes.txt" for a diagram about the descriptor area and the rings.) Because at most one descriptor chain can be in flight with VirtioLib at any time, - the Available Ring and the Used Ring proceed in lock-step, - and the head descriptor that the new "available" and "used" elements can ever reference has index 0. Based on the above, we can modify VirtioFlush() to return the number of bytes written by the host across the descriptor chain. The virtio-block and virtio-scsi drivers don't care (they have other ways to parse the data produced by the host), while the virtio-net driver doesn't use VirtioFlush() at all (it employs VirtioLib only to set up its rings). However, the virtio entropy device, to be covered in the upcoming patches, reports the amount of randomness produced by the host only through this quantity. Cc: Jordan Justen <jordan.l.justen@intel.com> Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>		2016-02-24 12:07:32 +01:00
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VirtioLib.c	OvmfPkg: VirtioFlush(): return the number of bytes written by the host	2016-02-24 12:07:32 +01:00
VirtioLib.inf	OvmfPkg: librarize reusable bits from VirtioBlkDxe's SynchronousRequest()	2012-10-12 18:54:35 +00:00