Re: [PATCH v5.5 20/30] KVM: x86: Use nr_memslot_pages to avoid traversing the memslots array
From: Maciej S. Szmigiero
Date: Tue Nov 09 2021 - 11:31:10 EST
On 09.11.2021 02:34, Sean Christopherson wrote:
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".
Unfortunately in this case (but fortunately in general) C has well-defined
unsigned overflow behavior so if this code has an overflow by its naive
reading the compiler is not allowed to transform it to a form which
doesn't have this overflow (as it will result in a different value).
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;
I agree this would be a good solution to our problem, since this macro is
unlikely to be converted into something user-configurable in the future.
So its units don't matter.
Thanks,
Maciej