google / alioth / 37579315330

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push

github

Committed by Lencerf

Commit Message

feat(vfio-user): implement VFIO_USER_DEVICE_GET_REGION_IO_FDS

Ask the server which sub-regions of a BAR it wants to watch with a file
descriptor, and register them as ioeventfds, so that a doorbell write no
longer travels through a VM exit and a REGION_WRITE round trip.

Device gains get_region_notifiers(), defaulting to none: kernel VFIO has
no equivalent, VFIO_DEVICE_IOEVENTFD goes the other way round and hands
an eventfd to the kernel so that it writes a fixed value to the device.
VfioPciDev::new() now takes an optional notifier registry, shared by the
BAR callbacks, which register the notifiers once the guest programs a
BAR address, mirroring what virtio-pci does for its queue notifiers.

Only VFIO_USER_IO_FD_TYPE_IOEVENTFD is honored. An ioregionfd needs a
KVM feature that is not upstream, and a shadow ioeventfd additionally
needs KVM to store the written value in a shared buffer, which upstream
cannot do either; libvfio-user documents clients as free to ignore it.
Sub-regions with KVM_IOEVENTFD_FLAG_PIO are dropped as well, since
NotifierRegistry cannot express port IO, as are sub-regions reaching
past the region, which the protocol makes the client's job to check.

Note the sub-region layout follows libvfio-user, which sends 48-byte
entries carrying the shadow fields whatever the type. The protocol
documentation describes 40-byte entries, predating shadow ioeventfds,
and computes the entry stride from an argsz that includes the message
header, which no implementation does.

Assisted-by: Antigravity:Claude-Opus-5
Signed-off-by: Changyuan Lyu <changyuanl@google.com>

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