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9p/xen: reconnect to the backend on resume - #36
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A request that does not fit in its ring sleeps in io_wait_event_killable() until the backend frees space, but the event handler wakes the ring with wake_up_interruptible(), which only wakes TASK_INTERRUPTIBLE sleepers. A TASK_KILLABLE waiter therefore sleeps until it is killed, which hangs a guest that has enough large requests in flight to fill a ring. Wake the ring with wake_up() so killable waiters see the freed space. Fixes: 9523fea ("net/9p: Switch to wait_event_killable()") Signed-off-by: Alex Zenla <alex@edera.dev>
The frontend's resume handler only warned that suspend and resume are unsupported. After a restore its rings, grant references and event channels are gone, yet the mounted client kept writing requests into them, so every request on the mount hung. The next InitWait from the backend then allocated a second frontend instance for the same device, which no client used. On resume, collect any replies the backend left in the rings, free the rings, fail the requests that are still unanswered with -EIO, and publish Initialising so that a backend which still serves the old rings starts over. When the backend reaches InitWait, allocate and publish new rings for the existing frontend, keeping its clients. Requests that arrive while the frontend is not connected wait until it is, and a rw_semaphore keeps them off the rings while a resume replaces them. Signed-off-by: Alex Zenla <alex@edera.dev>
A forked, restored or imported domain resumes against a new backend whose fid table is empty, while the guest's 9P clients still hold the fids of the session they were suspended with. With the transport reconnected every request on those fids still fails, so even an idle mount comes back unusable. When the backend advertises feature-session-transfer, wait at suspend for the requests in flight to be answered, then ask the backend for its fid table with Tsession_save and keep the pages in guest memory, where they survive into whatever domain the guest resumes in. Once a restored frontend's new backend connects, replay the pages with Tsession_restore before any request can use the rings. The pages are opaque to the frontend; the backend validates every entry as it would a walk or open. Signed-off-by: Alex Zenla <alex@edera.dev>
azenla
marked this pull request as ready for review
October 18, 2026 08:56
marly-edera
previously approved these changes
Oct 18, 2026
The toolstack has to know whether a guest can carry its 9P session into a new backend before it exports the domain, because a guest that cannot comes back with every open file on its mounts unusable. Until now only the backend advertised the feature, and the frontend's support was invisible from dom0. Publish feature-session-transfer = 1 in the frontend's own xenstore directory, next to version and num-rings. Signed-off-by: Alex Zenla <alex@edera.dev>
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9p/xen: reconnect to the backend on resume
A zone that is suspended and resumed, forked or restored comes back with its 9pfs rings gone. Today every access to the mount hangs.
-EIO, because whether they took effect is not known.wake_up(), so a waiter that sleeps killable is not missed.feature-session-transfer(an Edera extension, 9P ids 200 to 203), the frontend saves the backend's fid table at suspend and keeps it in guest memory. After a fork or restore it hands the table to the new backend, so open files keep working. The frontend does not read the saved data.Tested on Xen with a PVH zone and
protect-storagefrom edera-dev/protect (azenla/feat/9pfs-suspend-resume-storage). A workload held a file open on a 9pfs mount and wrote to it withsyncevery second.The frontend also publishes
feature-session-transferin its own xenstore node, so the toolstack can tell before it exports a zone whether the guest can hand its session back.