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Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting
To:     "Michael S. Tsirkin" <mst@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     Nitesh Narayan Lal <nitesh@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, lcapitulino@redhat.com,
        pagupta@redhat.com, wei.w.wang@intel.com,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        Rik van Riel <riel@surriel.com>, dodgen@google.com,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        dhildenb@redhat.com, Andrea Arcangeli <aarcange@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <0118482c-e633-3949-6797-94d7deb8a382@redhat.com>
Date:   Tue, 19 Feb 2019 21:02:52 +0100
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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.

> 
> All the issues are shared with balloon btw, so that
> could be another reason to use the balloon.

Yes, smells like it.

-- 

Thanks,

David / dhildenb