Public bug reported:
An unfortunate side effect of some of the recent work done to improve
ring buffer performance made a situation where the host would interpret
guest requests as a denial of service attack, pausing guest
communications for five seconds. This series of fixes corrects this
problem.
Needed for each kernel that has included a389fcfd2cb5 ("Drivers: hv:
vmbus: Fix signaling logic in hv_need_to_signal_on_read()")
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=74198eb4a42c4a3c4fbef08fa01a291a282f7c2e
Drivers: hv: vmbus: Base host signaling strictly on the ring state
One of the factors that can result in the host concluding that a given
guest in mounting a DOS attack is if the guest generates interrupts
to the host when the host is not expecting it. If these "spurious"
interrupts reach a certain rate, the host can throttle the guest to
minimize the impact. The host computation of the "expected number
of interrupts" is strictly based on the ring transitions. Until
the host logic is fixed, base the guest logic to interrupt solely
on the ring state.
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=1f6ee4e7d83586c8b10bd4f2f4346353d04ce884
Drivers: hv: vmbus: On write cleanup the logic to interrupt the host
Signal the host when we determine the host is to be signaled.
The currrent code determines the need to signal in the ringbuffer
code and actually issues the signal elsewhere. This can result
in the host viewing this interrupt as spurious since the host may also
poll the channel. Make the necessary adjustments.
https://git.kernel.org/cgit/linux/kernel/git/next/linux-
next.git/commit/?id=3372592a140db69fd63837e81f048ab4abf8111e
Drivers: hv: vmbus: On the read path cleanup the logic to interrupt the host
Signal the host when we determine the host is to be signaled -
on th read path. The currrent code determines the need to signal in the
ringbuffer code and actually issues the signal elsewhere. This can result
in the host viewing this interrupt as spurious since the host may also
poll the channel. Make the necessary adjustments.
https://git.kernel.org/cgit/linux/kernel/git/next/linux-
next.git/commit/?id=433e19cf33d34bb6751c874a9c00980552fe508c
Drivers: hv: vmbus: finally fix hv_need_to_signal_on_read()
Commit a389fcfd2cb5 ("Drivers: hv: vmbus: Fix signaling logic in
hv_need_to_signal_on_read()")
added the proper mb(), but removed the test "prev_write_sz < pending_sz"
when making the signal decision.
As a result, the guest can signal the host unnecessarily,
and then the host can throttle the guest because the host
thinks the guest is buggy or malicious; finally the user
running stress test can perceive intermittent freeze of
the guest.
This patch brings back the test, and properly handles the
in-place consumption APIs used by NetVSC (see get_next_pkt_raw(),
put_pkt_raw() and commit_rd_index()).
Fixes: a389fcfd2cb5 ("Drivers: hv: vmbus: Fix signaling logic in
hv_need_to_signal_on_read()")
** Affects: linux (Ubuntu)
Importance: Undecided
Status: Incomplete
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https://bugs.launchpad.net/bugs/1661430
Title:
[Hyper-V] Fix ring buffer handling to avoid host throttling
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