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

[David Hildenbrand <david@redhat.com>]

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.

-- 
Thanks,

David / dhildenb