On 16/04/2020, Roman Kagan wrote:
On Thu, Apr 16, 2020 at 11:38:46AM +0300, Jon Doron wrote:
According to the TLFS:
"A write to the end of message (EOM) register by the guest causes the
hypervisor to scan the internal message buffer queue(s) associated with
the virtual processor.
If a message buffer queue contains a queued message buffer, the hypervisor
attempts to deliver the message.
Message delivery succeeds if the SIM page is enabled and the message slot
corresponding to the SINTx is empty (that is, the message type in the
header is set to HvMessageTypeNone).
If a message is successfully delivered, its corresponding internal message
buffer is dequeued and marked free.
If the corresponding SINTx is not masked, an edge-triggered interrupt is
delivered (that is, the corresponding bit in the IRR is set).
This register can be used by guests to poll for messages. It can also be
used as a way to drain the message queue for a SINTx that has
been disabled (that is, masked)."
Doesn't this work already?
Well if you dont have SCONTROL and a GSI associated with the SINT then
it does not...
So basically this means that we need to exit on EOM so the hypervisor
will have a chance to send all the pending messages regardless of the
SCONTROL mechnaisim.
I might be misinterpreting the spec, but my understanding is that
SCONTROL {en,dis}ables the message queueing completely. What the quoted
part means is that a write to EOM should trigger the message source to
push a new message into the slot, regardless of whether the SINT was
masked or not.
And this (I think, haven't tested) should already work. The userspace
just keeps using the SINT route as it normally does, posting
notifications to the corresponding irqfd when posting a message, and
waiting on the resamplerfd for the message slot to become free. If the
SINT is masked KVM will skip injecting the interrupt, that's it.
Roman.
That's what I was thinking originally as well, but then i noticed KDNET
as a VMBus client (and it basically runs before anything else) is
working in this polling mode, where SCONTROL is disabled and it just
loops, and if it saw there is a PENDING message flag it will issue an
EOM to indicate it has free the slot.
(There are a bunch of patches i sent on the QEMU mailing list as well
where i CCed you, I will probably revise it a bit but was hoping to get
KVM sorted out first).
Cheers,
-- Jon.
