On Tue, Nov 09, 2021, Maciej S. Szmigiero wrote:
> On 04.11.2021 01:25, Sean Christopherson wrote:
> > From: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> >
> > There is no point in recalculating from scratch the total number of pages
> > in all memslots each time a memslot is created or deleted. Use KVM's
> > cached nr_memslot_pages to compute the default max number of MMU pages.
> >
> > Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> > [sean: use common KVM field and rework changelog accordingly]
Heh, and I forgot to add "and introduce bugs"
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> > arch/x86/include/asm/kvm_host.h | 1 -
> > arch/x86/kvm/mmu/mmu.c | 24 ------------------------
> > arch/x86/kvm/x86.c | 11 ++++++++---
> > 3 files changed, 8 insertions(+), 28 deletions(-)
> >
> (..)
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -11837,9 +11837,14 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> > enum kvm_mr_change change)
> > {
> > if (!kvm->arch.n_requested_mmu_pages &&
> > - (change == KVM_MR_CREATE || change == KVM_MR_DELETE))
> > - kvm_mmu_change_mmu_pages(kvm,
> > - kvm_mmu_calculate_default_mmu_pages(kvm));
> > + (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
> > + unsigned long nr_mmu_pages;
> > +
> > + nr_mmu_pages = kvm->nr_memslot_pages * KVM_PERMILLE_MMU_PAGES;
>
> Unfortunately, even if kvm->nr_memslot_pages is capped at ULONG_MAX then
> this value multiplied by 20 can still overflow an unsigned long variable.
Doh. And that likely subtly avoided by the compiler collapsing the "* 20 / 1000"
into "/ 50".
Any objection to adding a patch to cut out the multiplication entirely? Well, cut
it from the source code, looks like gcc generates some fancy SHR+MUL to do the
divide.
I'm thinking this:
#define KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO 50
...
nr_mmu_pages = nr_pages / KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO;
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