diff mbox series

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

Message ID 20211104002531.1176691-21-seanjc@google.com (mailing list archive)
State New, archived
Headers show
Series KVM: Scalable memslots implementation | expand

Commit Message

Sean Christopherson Nov. 4, 2021, 12:25 a.m. UTC 
From: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

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@oracle.com>
[sean: use common KVM field and rework changelog accordingly]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 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(-)

Comments

Maciej S. Szmigiero Nov. 9, 2021, 12:41 a.m. UTC | #1 
On 04.11.2021 01:25, Sean Christopherson wrote:
> From: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> 
> 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@oracle.com>
> [sean: use common KVM field and rework changelog accordingly]
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   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
Sean Christopherson Nov. 9, 2021, 1:34 a.m. UTC | #2 
On Tue, Nov 09, 2021, Maciej S. Szmigiero wrote:
> On 04.11.2021 01:25, Sean Christopherson wrote:
> > From: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> > 
> > 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@oracle.com>
> > [sean: use common KVM field and rework changelog accordingly]

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

> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >   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;
Maciej S. Szmigiero Nov. 9, 2021, 4:29 p.m. UTC | #3 
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@oracle.com>
>>>
>>> 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@oracle.com>
>>> [sean: use common KVM field and rework changelog accordingly]
> 
> Heh, and I forgot to add "and introduce bugs"
> 
>>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>>> ---
>>>    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
diff mbox series

Patch

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 88fce6ab4bbd..3fe155ece015 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1582,7 +1582,6 @@  void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
 				   const struct kvm_memory_slot *memslot);
 void kvm_mmu_zap_all(struct kvm *kvm);
 void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen);
-unsigned long kvm_mmu_calculate_default_mmu_pages(struct kvm *kvm);
 void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned long kvm_nr_mmu_pages);
 
 int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 354d2ca92df4..564781585fd2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6141,30 +6141,6 @@  int kvm_mmu_module_init(void)
 	return ret;
 }
 
-/*
- * Calculate mmu pages needed for kvm.
- */
-unsigned long kvm_mmu_calculate_default_mmu_pages(struct kvm *kvm)
-{
-	unsigned long nr_mmu_pages;
-	unsigned long nr_pages = 0;
-	struct kvm_memslots *slots;
-	struct kvm_memory_slot *memslot;
-	int i;
-
-	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
-		slots = __kvm_memslots(kvm, i);
-
-		kvm_for_each_memslot(memslot, slots)
-			nr_pages += memslot->npages;
-	}
-
-	nr_mmu_pages = nr_pages * KVM_PERMILLE_MMU_PAGES / 1000;
-	nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
-
-	return nr_mmu_pages;
-}
-
 void kvm_mmu_destroy(struct kvm_vcpu *vcpu)
 {
 	kvm_mmu_unload(vcpu);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4b0cb7390902..9a0440e22ede 100644
--- 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;
+		nr_mmu_pages /= 1000;
+		nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
+		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
+	}
 
 	kvm_mmu_slot_apply_flags(kvm, old, new, change);