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 09:34:25 -0800
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, rkrcmar@...hat.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 Sat, 2019-02-09 at 19:44 -0500, Michael S. Tsirkin wrote:
> On Mon, Feb 04, 2019 at 10:15:46AM -0800, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> > 
> > Add the host side of the KVM memory hinting support. With this we expose a
> > feature bit indicating that the host will pass the messages along to the
> > new madvise function.
> > 
> > This functionality is mutually exclusive with device assignment. If a
> > device is assigned we will disable the functionality as it could lead to a
> > potential memory corruption if a device writes to a page after KVM has
> > flagged it as not being used.
> 
> I really dislike this kind of tie-in.
> 
> 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 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.

Okay, I can do that. It also gives me a means of benchmarking just the
hypercall cost versus the extra page faults and zeroing.