On Sun, Feb 17, 2008 at 11:38:51AM +0200, Avi Kivity wrote:
> >+ * Return the pointer to the largepage write count for a given
> >+ * gfn, handling slots that are not large page aligned.
> >+ */
> >+static int *slot_largepage_idx(gfn_t gfn, struct kvm_memory_slot *slot)
> >+{
> >+ unsigned long idx;
> >+
> >+ idx = (gfn - slot->base_gfn) + hpage_align_diff(slot->base_gfn);
> >+ idx /= KVM_PAGES_PER_HPAGE;
> >+ return &slot->lpage_info[idx].write_count;
> >+}
> >
>
> Can be further simplified to (gfn / KVM_PAGES_PER_HPAGE) -
> (slot->base_gfn / KVM_PAGES_PER_HPAGE). Sorry for not noticing earlier.
Right.
> >+ /* guest has 4M pages, host 2M */
> >+ if (!is_pae(vcpu) && HPAGE_SHIFT == 21)
> >+ return 0;
> >
>
> Is this check necessary? I think that if we remove it things will just
> work. A 4MB page will be have either one or two 2MB sptes (which may
> even belong to different slots).
You mentioned that before, I agree its not necessary.
> >+ /*
> >+ * Largepage creation is susceptible to a upper-level
> >+ * table to be shadowed and write-protected in the
> >+ * area being mapped. If that is the case, invalidate
> >+ * the entry and let the instruction fault again
> >+ * and use 4K mappings.
> >+ */
> >+ if (largepage) {
> >+ spte = shadow_trap_nonpresent_pte;
> >+ kvm_x86_ops->tlb_flush(vcpu);
> >+ goto unshadowed;
> >+ }
> >
>
> Would it not repeat exactly the same code path? Or is this just for the
> case of the pte_update path?
The problem is if the instruction writing to one of the roots can't be
emulated.
kvm_mmu_unprotect_page() does not know about largepages, so it will zap
a gfn inside the large page frame, but not the large translation itself.
And zapping the gfn brings the shadowed page count in large area to
zero, allowing has_wrprotected_page() to succeed. Endless unfixable
write faults.
> >- page = gfn_to_page(vcpu->kvm, gpa >> PAGE_SHIFT);
> >+ if (is_largepage_backed(vcpu, gfn & ~(KVM_PAGES_PER_HPAGE-1))
> >+ && is_physical_memory(vcpu->kvm, gfn)) {
> >+ gfn &= ~(KVM_PAGES_PER_HPAGE-1);
> >+ largepage = 1;
> >+ }
> >
>
> Doesn't is_largepage_backed() imply is_physical_memory?
Hum. I'll verify... it seems that now that the ends of the slots have
write_count set to 1 that should be true.
> >
> >Index: kvm.largepages/arch/x86/kvm/x86.c
> >===================================================================
> >--- kvm.largepages.orig/arch/x86/kvm/x86.c
> >+++ kvm.largepages/arch/x86/kvm/x86.c
> >@@ -86,6 +86,7 @@ struct kvm_stats_debugfs_item debugfs_en
> > { "mmu_recycled", VM_STAT(mmu_recycled) },
> > { "mmu_cache_miss", VM_STAT(mmu_cache_miss) },
> > { "remote_tlb_flush", VM_STAT(remote_tlb_flush) },
> >+ { "lpages", VM_STAT(lpages) },
> > { NULL }
> > };
> >
>
> s/lpages/largepages/, this is user visible.
OK.
> >+ new.lpage_info = vmalloc(largepages *
> >sizeof(*new.lpage_info));
> >+
> >+ if (!new.lpage_info)
> >+ goto out_free;
> >+
> >+ memset(new.lpage_info, 0, largepages *
> >sizeof(*new.lpage_info));
> >+ /* large page crosses memslot boundary */
> >+ if (npages % KVM_PAGES_PER_HPAGE) {
> >+ new.lpage_info[0].write_count = 1;
> >
>
> This seems wrong, say a 3MB slot at 1GB, you kill the first largepage
> which is good.
>
> >+ new.lpage_info[largepages-1].write_count = 1;
> >
>
> OTOH, a 3MB slot at 3MB, the last page is fine. The check needs to be
> against base_gfn and base_gfn + npages, not the number of pages.
Right, will fix. Will post an uptodated patch soon.
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