Re: [PATCH v1 1/2] vhost: Defer filtering memory sections until building the vhost memory structure

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 16, 2023 at 01:13:07PM +0100, David Hildenbrand wrote:
> On 16.02.23 13:10, David Hildenbrand wrote:
> > On 16.02.23 13:04, Michael S. Tsirkin wrote:
> > > On Thu, Feb 16, 2023 at 12:47:51PM +0100, David Hildenbrand wrote:
> > > > Having multiple devices, some filtering memslots and some not filtering
> > > > memslots, messes up the "used_memslot" accounting. If we'd have a device
> > > > the filters out less memory sections after a device that filters out more,
> > > > we'd be in trouble, because our memslot checks stop working reliably.
> > > > For example, hotplugging a device that filters out less memslots might end
> > > > up passing the checks based on max vs. used memslots, but can run out of
> > > > memslots when getting notified about all memory sections.
> > > > 
> > > > Further, it will be helpful in memory device context in the near future
> > > > to know that a RAM memory region section will consume a memslot, and be
> > > > accounted for in the used vs. free memslots, such that we can implement
> > > > reservation of memslots for memory devices properly. Whether a device
> > > > filters this out and would theoretically still have a free memslot is
> > > > then hidden internally, making overall vhost memslot accounting easier.
> > > > 
> > > > Let's filter the memslots when creating the vhost memory array,
> > > > accounting all RAM && !ROM memory regions as "used_memslots" even if
> > > > vhost_user isn't interested in anonymous RAM regions, because it needs
> > > > an fd.
> > > > 
> > > > When a device actually filters out regions (which should happen rarely
> > > > in practice), we might detect a layout change although only filtered
> > > > regions changed. We won't bother about optimizing that for now.
> > > 
> > > That caused trouble in the past when using VGA because it is playing
> > > with mappings in weird ways.
> > > I think we have to optimize it, sorry.
> > 
> > We still filter them out, just later.
> 
> To be precise, we still filter out all DIRTY_MEMORY_VGA as we used to. Only
> the device-specific filtering (vhost-user) is modified.


Oh good so the VGA use-case is unaffected.

> -- 
> Thanks,
> 
> David / dhildenb