Re: [Xen-devel] PV Shim ballooning

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 11/02/2020 18:11, Roger Pau Monné wrote:
> On Tue, Feb 11, 2020 at 05:29:09PM +0000, Igor Druzhinin wrote:
>> On 11/02/2020 16:42, Roger Pau Monné wrote:
>>> On Tue, Feb 11, 2020 at 04:29:36PM +0000, Igor Druzhinin wrote:
>>>> Agree. But as I said I'm not aware of any guest that violates the
>>>> invariant of decrease_reservation() being the last call.
>>>
>>> Maybe we could piggyback on whether a page is removed from the domain
>>> domheap and use that as a signal that the page should be ballooned
>>> out?
>>>
>>> There's already an arch_free_heap_page that's called when a page is
>>> removed from a domain, which might be suitable for this. It would
>>> however imply making an hypercall for every page to be ballooned out.
>>
>> I tested that - doesn't work - too many hypercalls make ballooning take
>> ages. This simply cannot be done on page-by-page basis.
> 
> Why not place them on a list (in arch_free_heap_page) and do the flush
> either after a timeout or when it gets to a certain number of
> elements?

How do you know that "certain number of elements"? How do you know what
timeout is just right? This all seems like it will introduce more problems
than solve.

What if instead we place them on a temp list in L1 decrease_reservation()
and then will just move to another list as soon as they are freed,
then pass the whole list to L0 as soon as the first list is empty?

Igor

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