* [PATCH] KVM: MMU: update comment on the number of page role combinations
@ 2021-11-16 10:11 Paolo Bonzini
2021-11-16 11:07 ` Maxim Levitsky
0 siblings, 1 reply; 5+ messages in thread
From: Paolo Bonzini @ 2021-11-16 10:11 UTC (permalink / raw)
To: linux-kernel, kvm
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e977634333d4..354fd2a918d5 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -291,9 +291,9 @@ struct kvm_kernel_irq_routing_entry;
* the number of unique SPs that can theoretically be created is 2^n, where n
* is the number of bits that are used to compute the role.
*
- * But, even though there are 18 bits in the mask below, not all combinations
+ * But, even though there are 20 bits in the mask below, not all combinations
* of modes and flags are possible. The maximum number of possible upper-level
- * shadow pages for a single gfn is in the neighborhood of 2^13.
+ * shadow pages for a single gfn is in the neighborhood of 2^15.
*
* - invalid shadow pages are not accounted.
* - level is effectively limited to four combinations, not 16 as the number
--
2.27.0
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: MMU: update comment on the number of page role combinations
2021-11-16 10:11 [PATCH] KVM: MMU: update comment on the number of page role combinations Paolo Bonzini
@ 2021-11-16 11:07 ` Maxim Levitsky
2021-11-16 12:28 ` Paolo Bonzini
0 siblings, 1 reply; 5+ messages in thread
From: Maxim Levitsky @ 2021-11-16 11:07 UTC (permalink / raw)
To: Paolo Bonzini, linux-kernel, kvm
On Tue, 2021-11-16 at 05:11 -0500, Paolo Bonzini wrote:
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/include/asm/kvm_host.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index e977634333d4..354fd2a918d5 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -291,9 +291,9 @@ struct kvm_kernel_irq_routing_entry;
> * the number of unique SPs that can theoretically be created is 2^n, where n
> * is the number of bits that are used to compute the role.
> *
> - * But, even though there are 18 bits in the mask below, not all combinations
> + * But, even though there are 20 bits in the mask below, not all combinations
I to be honest counted 19 bits there (which includes the 'smm' bit), but I might have
made a mistake. I do wonder maybe it is better to just remove that comment with explicit
number?
Best regards,
Maxim Levitsky
> * of modes and flags are possible. The maximum number of possible upper-level
> - * shadow pages for a single gfn is in the neighborhood of 2^13.
> + * shadow pages for a single gfn is in the neighborhood of 2^15.
> *
> * - invalid shadow pages are not accounted.
> * - level is effectively limited to four combinations, not 16 as the number
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: MMU: update comment on the number of page role combinations
2021-11-16 11:07 ` Maxim Levitsky
@ 2021-11-16 12:28 ` Paolo Bonzini
2021-11-17 22:25 ` Sean Christopherson
0 siblings, 1 reply; 5+ messages in thread
From: Paolo Bonzini @ 2021-11-16 12:28 UTC (permalink / raw)
To: Maxim Levitsky, linux-kernel, kvm
On 11/16/21 12:07, Maxim Levitsky wrote:
>> - * But, even though there are 18 bits in the mask below, not all
>> combinations
>> + * But, even though there are 20 bits in the mask
>> below, not all combinations
> I to be honest counted 19 bits there (which includes the 'smm' bit),
> but I might have made a mistake. I do wonder maybe it is better to
> just remove that comment with explicit number?
Yes, they are 19. But the explicit number is there to guide in
understanding how 19 goes down to 14 combinations.
Here is a better writeup:
* - invalid shadow pages are not accounted, so the bits are effectively 18
* - quadrant will only be used if gpte_is_8_bytes is zero (non-PAE paging);
* execonly and ad_disabled are only used for nested EPT which has
* gpte_is_8_bytes=1. Therefore, 2 bits are always unused.
* - the 4 bits of level are effectively limited to the values 2/3/4/5,
* as 4k SPs are not tracked (allowed to go unsync). In addition non-PAE
* paging has exactly one upper level, making level effectively redundant
* when gpte_is_8_bytes=0.
* - on top of this, smep_andnot_wp and smap_andnot_wp are only set if cr0_wp=0,
* therefore these three bits only give rise to 5 possibilities.
FWIW, the full count becomes 6400 unless I screwed up the math.
Paolo
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: MMU: update comment on the number of page role combinations
2021-11-16 12:28 ` Paolo Bonzini
@ 2021-11-17 22:25 ` Sean Christopherson
2021-11-18 7:08 ` Paolo Bonzini
0 siblings, 1 reply; 5+ messages in thread
From: Sean Christopherson @ 2021-11-17 22:25 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: Maxim Levitsky, linux-kernel, kvm
On Tue, Nov 16, 2021, Paolo Bonzini wrote:
> On 11/16/21 12:07, Maxim Levitsky wrote:
> > > - * But, even though there are 18 bits in the mask below, not all
> > > combinations
> > > + * But, even though there are 20 bits in the mask
> > > below, not all combinations
> > I to be honest counted 19 bits there (which includes the 'smm' bit),
> > but I might have made a mistake. I do wonder maybe it is better to
> > just remove that comment with explicit number?
>
> Yes, they are 19. But the explicit number is there to guide in
No, there are 18 from a gfn_track perspective. "smm" isn't counted because it's
in a separate memslot address space. The "mask below" is definitely vague on that
point though.
> understanding how 19 goes down to 14 combinations.
>
> Here is a better writeup:
>
> * - invalid shadow pages are not accounted, so the bits are effectively 18
> * - quadrant will only be used if gpte_is_8_bytes is zero (non-PAE paging);
> * execonly and ad_disabled are only used for nested EPT which has
> * gpte_is_8_bytes=1. Therefore, 2 bits are always unused.
> * - the 4 bits of level are effectively limited to the values 2/3/4/5,
> * as 4k SPs are not tracked (allowed to go unsync). In addition non-PAE
> * paging has exactly one upper level, making level effectively redundant
> * when gpte_is_8_bytes=0.
> * - on top of this, smep_andnot_wp and smap_andnot_wp are only set if cr0_wp=0,
> * therefore these three bits only give rise to 5 possibilities.
>
> FWIW, the full count becomes 6400 unless I screwed up the math.
Which is "in the neighborhood of 2^13" :-)
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: MMU: update comment on the number of page role combinations
2021-11-17 22:25 ` Sean Christopherson
@ 2021-11-18 7:08 ` Paolo Bonzini
0 siblings, 0 replies; 5+ messages in thread
From: Paolo Bonzini @ 2021-11-18 7:08 UTC (permalink / raw)
To: Sean Christopherson; +Cc: Maxim Levitsky, linux-kernel, kvm
On 11/17/21 23:25, Sean Christopherson wrote:
>>
>> Here is a better writeup:
>>
>> * - invalid shadow pages are not accounted, so the bits are effectively 18
>> * - quadrant will only be used if gpte_is_8_bytes is zero (non-PAE paging);
>> * execonly and ad_disabled are only used for nested EPT which has
>> * gpte_is_8_bytes=1. Therefore, 2 bits are always unused.
>> * - the 4 bits of level are effectively limited to the values 2/3/4/5,
>> * as 4k SPs are not tracked (allowed to go unsync). In addition non-PAE
>> * paging has exactly one upper level, making level effectively redundant
>> * when gpte_is_8_bytes=0.
>> * - on top of this, smep_andnot_wp and smap_andnot_wp are only set if cr0_wp=0,
>> * therefore these three bits only give rise to 5 possibilities.
>>
>> FWIW, the full count becomes 6400 unless I screwed up the math.
> Which is "in the neighborhood of 2^13":-)
>
2^12 if SMM is not counted.
Paolo
^ permalink raw reply [flat|nested] 5+ messages in thread
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2021-11-16 10:11 [PATCH] KVM: MMU: update comment on the number of page role combinations Paolo Bonzini
2021-11-16 11:07 ` Maxim Levitsky
2021-11-16 12:28 ` Paolo Bonzini
2021-11-17 22:25 ` Sean Christopherson
2021-11-18 7:08 ` Paolo Bonzini
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