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    SubjectRe: [PATCH 2/3] KVM: MMU: don not retry #PF for nonpaging guest
    On Tue, Nov 09, 2010 at 12:58:24AM +0800, Xiao Guangrong wrote:
    > I mean the second case, the problem is in "d:", now, A-apf is on
    > L0->L1 mapping, and A-apf.gva is nested fault address(L2's physics
    > address in L0->L2 mapping), you use this address to retry #pf in
    > L0->L1 mapping? it is just waste.
    >
    No A-apf.gva should contain L1 physical address.

    > On 11/8/10, Gleb Natapov <gleb@redhat.com> wrote:
    > > On Mon, Nov 08, 2010 at 10:14:35AM +0800, Xiao Guangrong wrote:
    > >> On 11/05/2010 06:31 PM, Gleb Natapov wrote:
    > >> > On Fri, Nov 05, 2010 at 04:03:28PM +0800, Xiao Guangrong wrote:
    > >> >> On 11/05/2010 03:45 PM, Gleb Natapov wrote:
    > >> >>
    > >> >>>>
    > >> >>>> It looks like something broken: apfs can generated in L2 guest
    > >> >>>> (nested ntp guest)
    > >> >>>> and be retried in L1 guest.
    > >> >>>>
    > >> >>> Why is this a problem? apf will be generate on direct map even when L2
    > >> >>> guest is running so it should be OK to prefault it into direct map on
    > >> >>> completion.
    > >> >>>
    > >> >>
    > >> >> The nested_cr3 is different between L2 and L1, fix L2's page fault in
    > >> >> L1's page table
    > >> >> is useless.
    > >> > But we are fixing L0 page faults in L0 page table. We do not start apf
    > >> > because of L1 faulted in its page table.
    > >> >
    > >>
    > >> Hi Gleb,
    > >>
    > >> For example, NPT Guest L1 runs on Host, and Nested NPT Guest L2 runs on
    > >> Guest L1.
    > >> Now, Guest L2 is running, has below sequences:
    > >>
    > >> a: NPF/PF occurs in L2 Guest, and generates a apf(named A-apf), then
    > >> L2 Guest is blocked
    > >>
    > > What do you mean by that? Do you mean L2 exits to L1 with NPF because
    > > L1 swapped out L2 page? In this case apf will be generated and handled
    > > by L1 just like in L0->L1 non-nested case. Or do you mean L2 exits to L0
    > > with NPF because L0 swapped out L1 page? Lets assume you mean second
    > > case since it is interesting one.
    > >
    > >> b: a external event wakes up L2 Guest, and let it run again.
    > >>
    > >> c: L2 Guest VMEXIT to L1 Guest because L2 Guest's action is intercepted by
    > >> Guest L1
    > >>
    > >> d: When cpu enter L1 Guest, A-apf is completed, then it will retry A-apf
    > >> in
    > >> L1 Guest's mmu context, and this 'retry' is useless.
    > > Since A-apf was on L0->L1 mapping it is OK to prefault it into direct
    > > map. Wen L2 will run again and try to access this page it fill fault
    > > again since the page is not pre-faulted into nntp shadow page. This time
    > > L0 will find page in memory and will create shadow mapping for it
    > > without apf.
    > >
    > >>
    > >> Could you please point it out for me if i missed something. :-)
    > >>
    > >
    > > --
    > > Gleb.
    > > --
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    > > the body of a message to majordomo@vger.kernel.org
    > > More majordomo info at http://vger.kernel.org/majordomo-info.html
    > > Please read the FAQ at http://www.tux.org/lkml/
    > >
    >
    > --
    > Sent from my mobile device

    --
    Gleb.


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