On Thu, Mar 21, 2013 at 01:41:59PM +0800, Xiao Guangrong wrote:
> On 03/21/2013 04:14 AM, Marcelo Tosatti wrote:
> >
> > kvm_mmu_calculate_mmu_pages numbers,
> >
> > maximum number of shadow pages = 2% of mapped guest pages
> >
> > Does not make sense for TDP guests where mapping all of guest
> > memory with 4k pages cannot exceed "mapped guest pages / 512"
> > (not counting root pages).
> >
> > Allow that maximum for TDP, forcing the guest to recycle otherwise.
> >
> > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> >
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index 956ca35..a9694a8d7 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -4293,7 +4293,7 @@ nomem:
> > unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm)
> > {
> > unsigned int nr_mmu_pages;
> > - unsigned int nr_pages = 0;
> > + unsigned int i, nr_pages = 0;
> > struct kvm_memslots *slots;
> > struct kvm_memory_slot *memslot;
> >
> > @@ -4302,7 +4302,19 @@ unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm)
> > kvm_for_each_memslot(memslot, slots)
> > nr_pages += memslot->npages;
> >
> > - nr_mmu_pages = nr_pages * KVM_PERMILLE_MMU_PAGES / 1000;
> > + if (tdp_enabled) {
> > + /* one root page */
> > + nr_mmu_pages = 1;
> > + /* nr_pages / (512^i) per level, due to
> > + * guest RAM map being linear */
> > + for (i = 1; i < 4; i++) {
> > + int nr_pages_round = nr_pages + (1 << (9*i));
> > + nr_mmu_pages += nr_pages_round >> (9*i);
> > + }
>
> Marcelo,
>
> Can it work if nested guest is used? Did you see any problem in practice (direct guest
> uses more memory than your calculation)?
Direct guest can use more than the calculation by switching between
different paging modes.
About nested guest: at one point in time the working set cannot exceed
the number of physical pages visible by the guest.
Allowing an excessively high number of shadow pages is a security
concern, also, as unpreemptable long operations are necessary to tear
down the pages.
> And mmio also can build some page table that looks like not considered
> in this patch.
Right, but its only a few pages. Same argument as above: working set at
one given time is smaller than total RAM. Do you see any potential
problem?
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