Linux Virtualization - Nov 2017 - [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

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.

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 | 
REPORT_START";
     - 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 ACK.
     - 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.


Best,
Wei

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.

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?

> 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 |
> REPORT_START";
>     - 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
> ACK.
>     - 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.


How about we establish a baseline with a simple interface, and
then add the command register when it's actually benefitial.


> Best,
> Wei

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 |
>> REPORT_START";
>>      - 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
>> ACK.
>>      - 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).

Best,
Wei