linux-kernel - Re: [PATCH v16.1 6/9] virtio-balloon: Add support for providing free page reports to host

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Message-ID: <3a8d9e1a3a5528c3a0889448f2ffd02c186399b7.camel@linux.intel.com>
Date:   Tue, 11 Feb 2020 08:33:18 -0800
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     David Hildenbrand <david@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, willy@...radead.org,
        mhocko@...nel.org, linux-mm@...ck.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, vbabka@...e.cz,
        yang.zhang.wz@...il.com, nitesh@...hat.com, konrad.wilk@...cle.com,
        pagupta@...hat.com, riel@...riel.com, lcapitulino@...hat.com,
        dave.hansen@...el.com, wei.w.wang@...el.com, aarcange@...hat.com,
        pbonzini@...hat.com, dan.j.williams@...el.com, osalvador@...e.de
Subject: Re: [PATCH v16.1 6/9] virtio-balloon: Add support for providing
 free page reports to host

On Tue, 2020-02-11 at 16:13 +0100, David Hildenbrand wrote:
>  >> AFAIKs, the guest could inflate/deflate (esp. temporarily) and
> > > communicate via "actual" the actual balloon size as he sees it.
> > 
> > OK so you want hinted but unused pages counted, and reported
> > in "actual"? That's a vmexit before each page use ...
> 
> No, not at all. I rather meant, that it is unclear how
> inflation/deflation requests and "actual" *could* interact. Especially
> if we would consider free page reporting as some way of inflation
> (+immediate deflation) triggered by the guest. IMHO, we would not touch
> "actual" in that case.
> 
> But as I said, I am totally fine with keeping it as is in this patch.
> IOW not glue free page reporting to inflation/deflation but let it act
> like something different with its own semantics (and document these
> properly).
> 

Okay, so before I post v17 am I leaving the virtio-balloon changes as they
were then?

For what it is worth I agree with Michael that there is more to this than
just a scatter-gather queue. For now I am trying to keep the overall
impact on QEMU on the smaller side, and if we do end up supporting the
MADV_FREE instead of MADV_DONTNEED that would also have an impact on
things as it would be yet another difference between ballooning and
hinting.

Thanks.

- Alex