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

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Date:   Tue, 19 Feb 2019 15:17:40 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Rik van Riel <riel@...riel.com>, dodgen@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        dhildenb@...hat.com, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

On Tue, Feb 19, 2019 at 09:02:52PM +0100, David Hildenbrand wrote:
> 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.

OK but let's recognize it as a bug not a feature.

-- 
MST