Messages in this thread | | | Date | Wed, 15 Nov 2017 11:47:58 +0800 | From | Wei Wang <> | Subject | Re: [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ |
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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
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