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Date:   Thu, 5 Apr 2018 15:47:28 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "liliang.opensource@...il.com" <liliang.opensource@...il.com>,
        "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "quan.xu0@...il.com" <quan.xu0@...il.com>,
        "nilal@...hat.com" <nilal@...hat.com>,
        "riel@...hat.com" <riel@...hat.com>,
        "huangzhichao@...wei.com" <huangzhichao@...wei.com>
Subject: RE: [PATCH v30 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

On Thursday, April 5, 2018 10:04 PM, Michael S. Tsirkin wrote:
> On Thu, Apr 05, 2018 at 02:05:03AM +0000, Wang, Wei W wrote:
> > On Thursday, April 5, 2018 9:12 AM, Michael S. Tsirkin wrote:
> > > On Thu, Apr 05, 2018 at 12:30:27AM +0000, Wang, Wei W wrote:
> > > > On Wednesday, April 4, 2018 10:08 PM, Michael S. Tsirkin wrote:
> > > > > On Wed, Apr 04, 2018 at 10:07:51AM +0800, Wei Wang wrote:
> > > > > > On 04/04/2018 02:47 AM, Michael S. Tsirkin wrote:
> > > > > > > On Wed, Apr 04, 2018 at 12:10:03AM +0800, Wei Wang wrote:
> > > > I'm afraid the driver couldn't be aware if the added hints are
> > > > stale or not,
> > >
> > >
> > > No - I mean that driver has code that compares two values and stops
> > > reporting. Can one of the values be stale?
> >
> > The driver compares "vb->cmd_id_use != vb->cmd_id_received" to decide
> > if it needs to stop reporting hints, and cmd_id_received is what the
> > driver reads from host (host notifies the driver to read for the
> > latest value). If host sends a new cmd id, it will notify the guest to
> > read again. I'm not sure how that could be a stale cmd id (or maybe I
> > misunderstood your point here?)
> >
> > Best,
> > Wei
> 
> The comparison is done in one thread, the update in another one.

I think this isn't something that could be solved by adding a lock, unless host waits for the driver's ACK about finishing the update (this is not agreed in the QEMU part discussion).

Actually virtio_balloon has F_IOMMU_PLATFORM disabled, maybe we don't need to worry about that using DMA api case (we only have gpa added to the vq, and having some entries stay in the vq seems fine). For this feature, I think it would not work with F_IOMMU enabled either.

If there is any further need (I couldn't think of a need so far), I think we could consider to let host inject a vq interrupt at some point, and then the driver handler can do the virtqueue_get_buf work.

Best,
Wei

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