Re: [RFC PATCH 2/4] kvm: Add host side support for free memory hints

From: Michael S. Tsirkin
Date: Mon Feb 11 2019 - 12:48:46 EST

On Mon, Feb 11, 2019 at 09:41:19AM -0800, Dave Hansen wrote:
> On 2/9/19 4:44 PM, Michael S. Tsirkin wrote:
> > So the policy should not leak into host/guest interface.
> > Instead it is better to just keep the pages pinned and
> > ignore the hint for now.
> It does seems a bit silly to have guests forever hinting about freed
> memory when the host never has a hope of doing anything about it.
> Is that part fixable?

Yes just not with existing IOMMU APIs.

It's in the paragraph just above that you cut out:
Yes right now assignment is not smart enough but generally
you can protect the unused page in the IOMMU and that's it,
it's safe.

So e.g.
extern int iommu_remap(struct iommu_domain *domain, unsigned long iova,
phys_addr_t paddr, size_t size, int prot);

I can elaborate if you like but generally we would need an API that
allows you to atomically update a mapping for a specific page without
perturbing the mapping for other pages.