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

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Date:   Mon, 11 Feb 2019 12:48:35 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rkrcmar@...hat.com, alexander.h.duyck@...ux.intel.com,
        x86@...nel.org, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        pbonzini@...hat.com, tglx@...utronix.de, akpm@...ux-foundation.org
Subject: Re: [RFC PATCH 2/4] kvm: Add host side support for free memory hints

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.

-- 
MST