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

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]
> 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.

Thanks,
Maciej