Re: [PATCH v5.5 20/30] KVM: x86: Use nr_memslot_pages to avoid traversing the memslots array

On Tue, Nov 09, 2021, Maciej S. Szmigiero wrote:
> On 04.11.2021 01:25, Sean Christopherson wrote:
> > From: Maciej S. Szmigiero <[email protected]>
> > 
> > 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 <[email protected]>
> > [sean: use common KVM field and rework changelog accordingly]

Heh, and I forgot to add "and introduce bugs"

> > Signed-off-by: Sean Christopherson <[email protected]>
> > ---
> >   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;