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    SubjectRe: [RFC PATCH 1/1] Move two pinned pages to non-movable node in kvm.
    CCing Marcelo,

    On Wed, Jun 18, 2014 at 02:50:44PM +0800, Tang Chen wrote:
    > Hi Gleb,
    >
    > Thanks for the quick reply. Please see below.
    >
    > On 06/18/2014 02:12 PM, Gleb Natapov wrote:
    > >On Wed, Jun 18, 2014 at 01:50:00PM +0800, Tang Chen wrote:
    > >>[Questions]
    > >>And by the way, would you guys please answer the following questions for me ?
    > >>
    > >>1. What's the ept identity pagetable for ? Only one page is enough ?
    > >>
    > >>2. Is the ept identity pagetable only used in realmode ?
    > >> Can we free it once the guest is up (vcpu in protect mode)?
    > >>
    > >>3. Now, ept identity pagetable is allocated in qemu userspace.
    > >> Can we allocate it in kernel space ?
    > >What would be the benefit?
    >
    > I think the benefit is we can hot-remove the host memory a kvm guest
    > is using.
    >
    > For now, only memory in ZONE_MOVABLE can be migrated/hot-removed. And the
    > kernel
    > will never use ZONE_MOVABLE memory. So if we can allocate these two pages in
    > kernel space, we can pin them without any trouble. When doing memory
    > hot-remove,
    > the kernel will not try to migrate these two pages.
    But we can do that by other means, no? The patch you've sent for instance.

    >
    > >
    > >>
    > >>4. If I want to migrate these two pages, what do you think is the best way ?
    > >>
    > >I answered most of those here: http://www.mail-archive.com/kvm@vger.kernel.org/msg103718.html
    >
    > I'm sorry I must missed this email.
    >
    > Seeing your advice, we can unpin these two pages and repin them in the next
    > EPT violation.
    > So about this problem, which solution would you prefer, allocate these two
    > pages in kernel
    > space, or migrate them before memory hot-remove ?
    >
    > I think the first solution is simpler. But I'm not quite sure if there is
    > any other pages
    > pinned in memory. If we have the same problem with other kvm pages, I think
    > it is better to
    > solve it in the second way.
    >
    > What do you think ?
    Remove pinning is preferable. In fact looks like for identity pagetable
    it should be trivial, just don't pin. APIC access page is a little bit
    more complicated since its physical address needs to be tracked to be
    updated in VMCS.

    --
    Gleb.


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