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Re: [Qemu-devel] [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_P

From: Wei Wang
Subject: Re: [Qemu-devel] [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ
Date: Wed, 15 Nov 2017 11:47:58 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 11/15/2017 05:21 AM, Michael S. Tsirkin wrote:
On Tue, Nov 14, 2017 at 08:02:03PM +0800, Wei Wang wrote:
On 11/14/2017 01:32 AM, Michael S. Tsirkin wrote:
- guest2host_cmd: written by the guest to ACK to the host about the
commands that have been received. The host will clear the corresponding
bits on the host2guest_cmd register. The guest also uses this register
to send commands to the host (e.g. when finish free page reporting).
I am not sure what is the role of guest2host_cmd. Reporting of
the correct cmd id seems sufficient indication that guest
received the start command. Not getting any more seems sufficient
to detect stop.

I think the issue is when the host is waiting for the guest to report pages,
it does not know whether the guest is going to report more or the report is
done already. That's why we need a way to let the guest tell the host "the
report is done, don't wait for more", then the host continues to the next
step - sending the non-free pages to the destination. The following method
is a conclusion of other comments, with some new thought. Please have a
check if it is good.
config won't work well for this IMHO.
Writes to config register are hard to synchronize with the VQ.
For example, guest sends free pages, host says stop, meanwhile
guest sends stop for 1st set of pages.

I still don't see an issue with this. Please see below:
(before jumping into the discussion, just make sure I've well explained this point: now host-to-guest commands are done via config, and guest-to-host commands are done via the free page vq)

Case: Host starts to request the reporting with cmd_id=1. Some time later, Host writes "stop" to config, meantime guest happens to finish the reporting and plan to actively send a "stop" command from the free_page_vq(). Essentially, this is like a sync between two threads - if we view the config interrupt handler as one thread, another is the free page reporting worker thread.

        - what the config handler does is simply:
              1.1:  WRITE_ONCE(vb->reporting_stop, true);

        - what the reporting thread will do is
              2.1:  WRITE_ONCE(vb->reporting_stop, true);
              2.2:  send_stop_to_host_via_vq();

From the guest point of view, no matter 1.1 is executed first or 2.1 first, it doesn't make a difference to the end result - vb->reporting_stop is set.

From the host point of view, it knows that cmd_id=1 has truly stopped the reporting when it receives a "stop" sign via the vq.

How about adding a buffer with "stop" in the VQ instead?
Wastes a VQ entry which you will need to reserve for this
but is it a big deal?

The free page vq is guest-to-host direction. Using it for host-to-guest requests will make it bidirectional, which will result in the same issue described before: https://lkml.org/lkml/2017/10/11/1009 (the first response)

On the other hand, I think adding another new vq for host-to-guest requesting doesn't make a difference in essence, compared to using config (same 1.1, 2.1, 2.2 above), but will be more complicated.

Two new configuration registers in total:
- cmd_reg: the command register, combined from the previous host2guest and
guest2host. I think we can use the same register for host requesting and
guest ACKing, since the guest writing will trap to QEMU, that is, all the
writes to the register are performed in QEMU, and we can keep things work in
a correct way there.
- cmd_id_reg: the sequence id of the free page report command.

-- free page report:
     - host requests the guest to start reporting by "cmd_reg |
     - guest ACKs to the host about receiving the start reporting request by
"cmd_reg | REPORT_START", host will clear the flag bit once receiving the
     - host requests the guest to stop reporting by "cmd_reg | REPORT_STOP";
     - guest ACKs to the host about receiving the stop reporting request by
"cmd_reg | REPORT_STOP", host will clear the flag once receiving the ACK.
     - guest tells the host about the start of the reporting by writing "cmd
id" into an outbuf, which is added to the free page vq.
     - guest tells the host about the end of the reporting by writing "0"
into an outbuf, which is added to the free page vq. (we reserve "id=0" as
the stop sign)

-- ballooning:
     - host requests the guest to start ballooning by "cmd_reg | BALLOONING";
     - guest ACKs to the host about receiving the request by "cmd_reg |
BALLOONING", host will clear the flag once receiving the ACK.

Some more explanations:
-- Why not let the host request the guest to start the free page reporting
simply by writing a new cmd id to the cmd_id_reg?
The configuration interrupt is shared among all the features - ballooning,
free page reporting, and future feature extensions which need host-to-guest
requests. Some features may need to add other feature specific configuration
registers, like free page reporting need the cmd_id_reg, which is not used
by ballooning. The rule here is that the feature specific registers are read
only when that feature is requested via the cmd_reg. For example, the
cmd_id_reg is read only when "cmd_reg | REPORT_START" is true. Otherwise,
when the driver receives a configuration interrupt, it has to read both
cmd_reg and cmd_id registers to know what are requested by the host - think
about the case that ballooning requests are sent frequently while free page
reporting isn't requested, the guest has to read the cmd_id register every
time a ballooning request is sent by the host, which is not necessary. If
future new features follow this style, there will be more unnecessary
VMexits to read the unused feature specific registers.
So I think it is good to have a central control of the feature request via
only one cmd register - reading that one is enough to know what is requested
by the host.

Right now you are increasing the cost of balloon request 3x though.

Not that much, I think, just a cmd register read and ACK, and this should be neglected compared to the ballooning time.
(I don't see a difference in the performance testing either).


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