Messages in this thread | | | Date | Thu, 16 Nov 2017 19:59:22 +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 09:26 PM, Michael S. Tsirkin wrote: > On Wed, Nov 15, 2017 at 11:47:58AM +0800, Wei Wang wrote: >> 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) > This is fine by me actually. But right now you have guest to host > not going through vq, going through command register instead - > this is how sending stop to host seems to happen. > If you make it go through vq then I think all will be well. > >> 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. > Yes, for guest to host stop sign. > >> 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. > I agree with this. Host to guest can just incremenent the "free command id" > register.
OK, thanks for the suggestions. I think one more issue left here:
Previously, when the guest receives a config interrupt, it blindly adds the balloon work item to the workqueue in virtballoon_changed(), because only ballooning uses the config. Now, free page reporting is requested via config, too.
We have the following two options:
Option 1: add "diff = towards_target()" to virtballoon_changed(), and if diff = 0, it will not add the balloon work item to the wq.
Option 2: add "cmd" for the host-to-guest request, and add the item when "cmd | CMD_BALLOON" is true.
I'm inclined to take option 1 now. Which one would you prefer?
Best, Wei
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