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| author | Thanos Makatos <thanos.makatos@nutanix.com> | 2022-07-04 12:16:08 +0100 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2022-07-04 12:16:08 +0100 |
| commit | 36beb63be45ad1412562a98d9373a4c0bd91ab3d (patch) | |
| tree | 61bb44f0befc0055292b120909251c0fc2d27b0e /docs | |
| parent | 1c274027bb4f9d68eee846036e8d50dcde2fd7e9 (diff) | |
| download | libvfio-user-36beb63be45ad1412562a98d9373a4c0bd91ab3d.zip libvfio-user-36beb63be45ad1412562a98d9373a4c0bd91ab3d.tar.gz libvfio-user-36beb63be45ad1412562a98d9373a4c0bd91ab3d.tar.bz2 | |
support for shadow ioeventfd (#698)
When an ioeventfd is written to, KVM discards the value since it has no
memory to write it to, and simply kicks the eventfd. This a problem for
devices such a NVMe controllers that need the value (e.g. doorbells on
BAR0). This patch allows the vfio-user server to pass a file descriptor
that can be mmap'ed and KVM can write the ioeventfd value to this
_shadow_ memory instead of discarding it. This shadow memory is not
exposed to the guest.
Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
Reviewed-by: John Levon <john.levon@nutanix.com>
Change-Id: Iad849c94076ffa5988e034c8bf7ec312d01f095f
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/ioregionfd.md | 25 |
1 files changed, 25 insertions, 0 deletions
diff --git a/docs/ioregionfd.md b/docs/ioregionfd.md new file mode 100644 index 0000000..c09b077 --- /dev/null +++ b/docs/ioregionfd.md @@ -0,0 +1,25 @@ +# ioregionfd + +ioregionfd is a mechanism that speeds up ioeventfds: +https://lore.kernel.org/kvm/cover.1613828726.git.eafanasova@gmail.com/. In the +author's original words: "ioregionfd is a KVM dispatch mechanism which can be +used for handling MMIO/PIO accesses over file descriptors without returning +from ioctl(KVM_RUN).". + +libvfio-user currently supports an experimental variant of this mechanism +called shadow ioeventfd. A shadow ioeventfd is a normal ioeventfd where the +vfio-user server passes another piece of memory (called the _shadow_ memory) +via an additional file descriptor when configuring the ioregionfd, which then +QEMU memory maps and passes this address to KVM. This shadow memory is never +exposed to the guest. When the guest writes to the trapped memory, KVM writes +the value to the shadow memory instread of discarding it, and then proceeds +kicking the eventfd as normal. + +To use shadow ioeventfd, the kernel and QEMU need to be patched. The QEMU patch +is designed specifically for SPDK's doorbells (one ioregionfd of 4K in BAR0); +it should be trivial to extend. + +The list of patches: +* kernel: https://gist.github.com/tmakatos/532afd092a8df2175120d3dbfcd719ef +* QEMU: https://gist.github.com/tmakatos/57755d2a37a6d53c9ff392e7c34470f6 +* SPDK: https://gist.github.com/tmakatos/f6c10fdaff59c9d629f94bd8e44a53bc |