Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

[David Hildenbrand]

On 19.02.19 20:58, Michael S. Tsirkin wrote:
> On Tue, Feb 19, 2019 at 10:06:35AM -0800, Alexander Duyck wrote:
>>> I tend to like an asynchronous reporting approach as discussed in this
>>> thread, we would have to see if Nitesh could get it implemented.
>>
>> I agree it would be great if it could work. However I have concerns
>> given that work on this patch set dates back to 2017, major issues
>> such as working around device assignment have yet to be addressed,
> 
> BTW for device assignment to work, your idea of sending
> data directly to kvm won't work, will it?
> You need to update userspace so it can update VFIO right?
> Another blocker for assignment is ability to make holes
> an an existing mapping - supported by hardware but
> not by IOMMU drivers.

I had the exact same thought and then realized that we decided to block
the balloon in user space until we figured out how to handle this properly.

I wonder if MADV_FREE behaves differently compared to MADV_DONTNEED when
finding pinned pages, but I doubt it. Most probably we'll have to
disable hinting for device assignments as well.

> 
> All the issues are shared with balloon btw, so that
> could be another reason to use the balloon.

Yes, smells like it.

-- 

Thanks,

David / dhildenb