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

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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



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