linux-kernel - Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting"

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Date:   Wed, 24 Jul 2019 14:14:43 -0700
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>, nitesh@...hat.com,
        kvm@...r.kernel.org, david@...hat.com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, yang.zhang.wz@...il.com,
        pagupta@...hat.com, riel@...riel.com, konrad.wilk@...cle.com,
        lcapitulino@...hat.com, wei.w.wang@...el.com, aarcange@...hat.com,
        pbonzini@...hat.com, dan.j.williams@...el.com
Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble
 hinting"

On Wed, 2019-07-24 at 16:46 -0400, Michael S. Tsirkin wrote:
> On Wed, Jul 24, 2019 at 01:18:00PM -0700, Alexander Duyck wrote:
> > On Wed, 2019-07-24 at 15:02 -0400, Michael S. Tsirkin wrote:
> > > On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote:
> > > > From: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> > > > 
> > > > Add support for what I am referring to as "bubble hinting". Basically the
> > > > idea is to function very similar to how the balloon works in that we
> > > > basically end up madvising the page as not being used. However we don't
> > > > really need to bother with any deflate type logic since the page will be
> > > > faulted back into the guest when it is read or written to.
> > > > 
> > > > This is meant to be a simplification of the existing balloon interface
> > > > to use for providing hints to what memory needs to be freed. I am assuming
> > > > this is safe to do as the deflate logic does not actually appear to do very
> > > > much other than tracking what subpages have been released and which ones
> > > > haven't.
> > > > 
> > > > Signed-off-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> > > > ---
> > > >  hw/virtio/virtio-balloon.c                      |   40 +++++++++++++++++++++++
> > > >  include/hw/virtio/virtio-balloon.h              |    2 +
> > > >  include/standard-headers/linux/virtio_balloon.h |    1 +
> > > >  3 files changed, 42 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > > > index 2112874055fb..70c0004c0f88 100644
> > > > --- a/hw/virtio/virtio-balloon.c
> > > > +++ b/hw/virtio/virtio-balloon.c
> > > > @@ -328,6 +328,39 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
> > > >      balloon_stats_change_timer(s, 0);
> > > >  }
> > > >  
> > > > +static void virtio_bubble_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> > > > +{
> > > > +    VirtQueueElement *elem;
> > > > +
> > > > +    while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) {
> > > > +    	unsigned int i;
> > > > +
> > > > +        for (i = 0; i < elem->in_num; i++) {
> > > > +            void *addr = elem->in_sg[i].iov_base;
> > > > +            size_t size = elem->in_sg[i].iov_len;
> > > > +            ram_addr_t ram_offset;
> > > > +            size_t rb_page_size;
> > > > +            RAMBlock *rb;
> > > > +
> > > > +            if (qemu_balloon_is_inhibited())
> > > > +                continue;
> > > > +
> > > > +            rb = qemu_ram_block_from_host(addr, false, &ram_offset);
> > > > +            rb_page_size = qemu_ram_pagesize(rb);
> > > > +
> > > > +            /* For now we will simply ignore unaligned memory regions */
> > > > +            if ((ram_offset | size) & (rb_page_size - 1))
> > > > +                continue;
> > > > +
> > > > +            ram_block_discard_range(rb, ram_offset, size);
> > > 
> > > I suspect this needs to do like the migration type of
> > > hinting and get disabled if page poisoning is in effect.
> > > Right?
> > 
> > Shouldn't something like that end up getting handled via
> > qemu_balloon_is_inhibited, or did I miss something there? I assumed cases
> > like that would end up setting qemu_balloon_is_inhibited to true, if that
> > isn't the case then I could add some additional conditions. I would do it
> > in about the same spot as the qemu_balloon_is_inhibited check.
> 
> Well qemu_balloon_is_inhibited is for the regular ballooning,
> mostly a work-around for limitations is host linux iommu
> APIs when it's used with VFIO.

I understood that. However it also addresses the shared memory case as
well if I recall correctly. Basically any case where us discarding the
page could cause issues we should be causing that function to return true.