Messages in this thread |  | | | Date | Fri, 12 Mar 2010 09:36:41 +0200 | | From | Avi Kivity <> | | Subject | Re: [PATCH 0/18][RFC] Nested Paging support for Nested SVM (aka NPT-Virtualization) |
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On 03/11/2010 10:58 PM, Marcelo Tosatti wrote: > >>> Can't you translate l2_gpa -> l1_gpa walking the current l1 nested >>> pagetable, and pass that to the kvm tdp fault path (with the correct >>> context setup)? >>> >> If I understand your suggestion correctly, I think thats exactly whats >> done in the patches. Some words about the design: >> >> For nested-nested we need to shadow the l1-nested-ptable on the host. >> This is done using the vcpu->arch.mmu context which holds the l1 paging >> modes while the l2 is running. On a npt-fault from the l2 we just >> instrument the shadow-ptable code. This is the common case. because it >> happens all the time while the l2 is running. >> > OK, makes sense now, I was missing the fact that the l1-nested-ptable > needs to be shadowed and l1 translations to it must be write protected. >
Shadow converts (gva -> gpa -> hpa) to (gva -> hpa) or (ngpa -> gpa -> hpa) to (ngpa -> hpa) equally well. In the second case npt still does (ngva -> ngpa). > You should disable out of sync shadow so that l1 guest writes to > l1-nested-ptables always trap.
Why? The guest is under obligation to flush the tlb if it writes to a page table, and we will resync on that tlb flush.
Unsync makes just as much sense for nnpt. Think of khugepaged in the guest eating a page table and spitting out a PDE.
> And in the trap case, you'd have to > invalidate l2 shadow pagetable entries that used the (now obsolete) > l1-nested-ptable entry. Does that happen automatically? >
What do you mean by 'l2 shadow ptable entries'? There are the guest's page tables (ordinary direct mapped, unless the guest's guest is also running an npt-enabled hypervisor), and the host page tables. When the guest writes to each page table, we invalidate the shadows.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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