On Sat, Mar 28, 2009 at 06:28:35PM -0300, Marcelo Tosatti wrote:
> > I have searched this bug for quite some time with no real luck. Maybe
> > some other reviewers have more luck than I had by now.
>
> Sorry, I can't spot what is wrong here. Avi?
>
> Perhaps it helps if you provide some info of the hang when guest
> allocates hugepages on boot (its probably and endless fault that can't
> be corrected?).
I will try to find out why the guest stucks. I also created a full
mmu trace of a boot crash case. But its size was around 170MB and I
found no real problem there.
> Also another point is that the large huge page at 0-1GB will never
> be created, because it crosses slot boundary.
The instabilies only occur if the guest has enough memory to use a
gbpage in its own direct mapping. They also go away when I boot it with
nogbpages command line option. So its likely that is has something to do
with the processing of guest gbpages in the softmmu code. But I looked
over this code again and again. There seems to be no bug.
>
> > Signed-off-by: Joerg Roedel <joerg.roedel@xxxxxxx>
> > ---
> > arch/x86/kvm/mmu.c | 56 +++++++++++++++++++++++++++++++------------
> > arch/x86/kvm/paging_tmpl.h | 35 +++++++++++++++++++++------
> > arch/x86/kvm/svm.c | 2 +-
> > 3 files changed, 68 insertions(+), 25 deletions(-)
> >
> > + psize = backing_size(vcpu, vcpu->arch.update_pte.gfn);
>
> This can block, and this path holds mmu_lock. Thats why it needs to be
> done in guess_page_from_pte_write.
Ah true. Thanks for pointing this out. The previous code in the
guess_page function makes sense now.
> > + if ((sp->role.level == PT_DIRECTORY_LEVEL) &&
> > + (psize >= KVM_PAGE_SIZE_2M)) {
> > + psize = KVM_PAGE_SIZE_2M;
> > + vcpu->arch.update_pte.gfn &= ~(KVM_PAGES_PER_2M_PAGE-1);
> > + vcpu->arch.update_pte.pfn &= ~(KVM_PAGES_PER_2M_PAGE-1);
> > + } else if ((sp->role.level == PT_MIDDLE_LEVEL) &&
> > + (psize == KVM_PAGE_SIZE_1G)) {
> > + vcpu->arch.update_pte.gfn &= ~(KVM_PAGES_PER_1G_PAGE-1);
> > + vcpu->arch.update_pte.pfn &= ~(KVM_PAGES_PER_1G_PAGE-1);
> > + } else
> > + goto out_pde;
>
> Better just zap the entry in case its a 1GB one and let the
> fault path handle it.
Yes, that probably better.
Joerg
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