diff mbox series

[v2] KVM: x86/pmu: Make top-down.slots event unavailable in supported leaf

Message ID 20220106032118.34459-1-likexu@tencent.com (mailing list archive)
State New, archived
Headers show
Series [v2] KVM: x86/pmu: Make top-down.slots event unavailable in supported leaf | expand

Commit Message

Like Xu Jan. 6, 2022, 3:21 a.m. UTC 
From: Like Xu <likexu@tencent.com>

When we choose to disable the fourth fixed counter TOPDOWN.SLOTS,
we also need to comply with the specification and set 0AH.EBX.[bit 7]
to 1 if the guest (e.g. on the ICX) has a value of 0AH.EAX[31:24] > 7.

Fixes: 2e8cd7a3b8287 ("kvm: x86: limit the maximum number of vPMU fixed counters to 3")
Signed-off-by: Like Xu <likexu@tencent.com>
---
v1 -> v2 Changelog:
- Make it simpler to keep forward compatibility; (Sean)
- Wrap related comment at ~80 chars; (Sean)

Previous:
https://lore.kernel.org/kvm/20220105050711.67280-1-likexu@tencent.com/

 arch/x86/kvm/cpuid.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

Comments

Sean Christopherson Jan. 6, 2022, 6:09 p.m. UTC | #1 
On Thu, Jan 06, 2022, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> When we choose to disable the fourth fixed counter TOPDOWN.SLOTS,
> we also need to comply with the specification and set 0AH.EBX.[bit 7]
> to 1 if the guest (e.g. on the ICX) has a value of 0AH.EAX[31:24] > 7.
> 
> Fixes: 2e8cd7a3b8287 ("kvm: x86: limit the maximum number of vPMU fixed counters to 3")
> Signed-off-by: Like Xu <likexu@tencent.com>
> ---
> v1 -> v2 Changelog:
> - Make it simpler to keep forward compatibility; (Sean)
> - Wrap related comment at ~80 chars; (Sean)
> 
> Previous:
> https://lore.kernel.org/kvm/20220105050711.67280-1-likexu@tencent.com/
> 
>  arch/x86/kvm/cpuid.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0b920e12bb6d..4fe17a537084 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -782,6 +782,18 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		eax.split.mask_length = cap.events_mask_len;
>  
>  		edx.split.num_counters_fixed = min(cap.num_counters_fixed, MAX_FIXED_COUNTERS);
> +
> +		/*
> +		 * The 8th Intel architectural event (Topdown Slots) will be supported

Nit, the "8th" part is unnecessary information.  

> +		 * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.
> +		 *
> +		 * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
> +		 * to make this event unavailable in a consistent way.
> +		 */

This comment is now slightly stale.  It also doesn't say why the event is made
unavailable.

> +		if (edx.split.num_counters_fixed < 4 &&

Rereading the changelog and the changelog of the Fixed commit, I don't think KVM
should bother checking num_counters_fixed.  IIUC, cap.events_mask[7] should already
be '1' if there are less than 4 fixed counters in hardware, but at the same time
there's no harm in being a bit overzealous.  That would help simplifiy the comment
as there's no need to explain why num_counters_fixed is checked, e.g. the fact that
Topdown Slots uses the 4th fixed counter is irrelevant with respect to the legality
of setting EBX[7]=1 to hide an unsupported event.

		/*
		 * Hide Intel's Topdown Slots architectural event, it's not yet
		 * supported by KVM.
		 */
		if (eax.split.mask_length > 7)
			cap.events_mask |= BIT_ULL(7);

> +		    eax.split.mask_length > 7)
> +			cap.events_mask |= BIT_ULL(7);
> +
>  		edx.split.bit_width_fixed = cap.bit_width_fixed;
>  		if (cap.version)
>  			edx.split.anythread_deprecated = 1;
> -- 
> 2.33.1
>
Jim Mattson Jan. 6, 2022, 8:12 p.m. UTC | #2 
On Thu, Jan 6, 2022 at 10:09 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Thu, Jan 06, 2022, Like Xu wrote:
> > From: Like Xu <likexu@tencent.com>
> >
> > When we choose to disable the fourth fixed counter TOPDOWN.SLOTS,
> > we also need to comply with the specification and set 0AH.EBX.[bit 7]
> > to 1 if the guest (e.g. on the ICX) has a value of 0AH.EAX[31:24] > 7.
> >
> > Fixes: 2e8cd7a3b8287 ("kvm: x86: limit the maximum number of vPMU fixed counters to 3")
> > Signed-off-by: Like Xu <likexu@tencent.com>
> > ---
> > v1 -> v2 Changelog:
> > - Make it simpler to keep forward compatibility; (Sean)
> > - Wrap related comment at ~80 chars; (Sean)
> >
> > Previous:
> > https://lore.kernel.org/kvm/20220105050711.67280-1-likexu@tencent.com/
> >
> >  arch/x86/kvm/cpuid.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 0b920e12bb6d..4fe17a537084 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -782,6 +782,18 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
> >               eax.split.mask_length = cap.events_mask_len;
> >
> >               edx.split.num_counters_fixed = min(cap.num_counters_fixed, MAX_FIXED_COUNTERS);
> > +
> > +             /*
> > +              * The 8th Intel architectural event (Topdown Slots) will be supported
>
> Nit, the "8th" part is unnecessary information.
>
> > +              * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.
> > +              *
> > +              * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
> > +              * to make this event unavailable in a consistent way.
> > +              */
>
> This comment is now slightly stale.  It also doesn't say why the event is made
> unavailable.
>
> > +             if (edx.split.num_counters_fixed < 4 &&
>
> Rereading the changelog and the changelog of the Fixed commit, I don't think KVM
> should bother checking num_counters_fixed.  IIUC, cap.events_mask[7] should already
> be '1' if there are less than 4 fixed counters in hardware, but at the same time
> there's no harm in being a bit overzealous.  That would help simplifiy the comment
> as there's no need to explain why num_counters_fixed is checked, e.g. the fact that
> Topdown Slots uses the 4th fixed counter is irrelevant with respect to the legality
> of setting EBX[7]=1 to hide an unsupported event.

I was under the impression that the CPUID.0AH:EBX bits were
independent of the fixed counters. That is, if CPUID.0AH:EAX[31:24] >
7 and CPUID.0AH:EBX[7] is clear, then one should be able to program a
general purpose counter with event selector A4H and umask 01H,
regardless of whether or not fixed counter 4 exists.

>
>                 /*
>                  * Hide Intel's Topdown Slots architectural event, it's not yet
>                  * supported by KVM.
>                  */
>                 if (eax.split.mask_length > 7)
>                         cap.events_mask |= BIT_ULL(7);
>
> > +                 eax.split.mask_length > 7)
> > +                     cap.events_mask |= BIT_ULL(7);
> > +
> >               edx.split.bit_width_fixed = cap.bit_width_fixed;
> >               if (cap.version)
> >                       edx.split.anythread_deprecated = 1;
> > --
> > 2.33.1
> >
Like Xu Jan. 7, 2022, 4:13 a.m. UTC | #3 
On 7/1/2022 4:12 am, Jim Mattson wrote:
> On Thu, Jan 6, 2022 at 10:09 AM Sean Christopherson <seanjc@google.com> wrote:
>>
>> On Thu, Jan 06, 2022, Like Xu wrote:
>>> From: Like Xu <likexu@tencent.com>
>>>
>>> When we choose to disable the fourth fixed counter TOPDOWN.SLOTS,
>>> we also need to comply with the specification and set 0AH.EBX.[bit 7]
>>> to 1 if the guest (e.g. on the ICX) has a value of 0AH.EAX[31:24] > 7.
>>>
>>> Fixes: 2e8cd7a3b8287 ("kvm: x86: limit the maximum number of vPMU fixed counters to 3")
>>> Signed-off-by: Like Xu <likexu@tencent.com>
>>> ---
>>> v1 -> v2 Changelog:
>>> - Make it simpler to keep forward compatibility; (Sean)
>>> - Wrap related comment at ~80 chars; (Sean)
>>>
>>> Previous:
>>> https://lore.kernel.org/kvm/20220105050711.67280-1-likexu@tencent.com/
>>>
>>>   arch/x86/kvm/cpuid.c | 12 ++++++++++++
>>>   1 file changed, 12 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index 0b920e12bb6d..4fe17a537084 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -782,6 +782,18 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>>>                eax.split.mask_length = cap.events_mask_len;
>>>
>>>                edx.split.num_counters_fixed = min(cap.num_counters_fixed, MAX_FIXED_COUNTERS);
>>> +
>>> +             /*
>>> +              * The 8th Intel architectural event (Topdown Slots) will be supported
>>
>> Nit, the "8th" part is unnecessary information.
>>
>>> +              * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.
>>> +              *
>>> +              * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
>>> +              * to make this event unavailable in a consistent way.
>>> +              */
>>
>> This comment is now slightly stale.  It also doesn't say why the event is made
>> unavailable.
>>
>>> +             if (edx.split.num_counters_fixed < 4 &&
>>
>> Rereading the changelog and the changelog of the Fixed commit, I don't think KVM
>> should bother checking num_counters_fixed.  IIUC, cap.events_mask[7] should already
>> be '1' if there are less than 4 fixed counters in hardware, but at the same time
>> there's no harm in being a bit overzealous.  That would help simplifiy the comment
>> as there's no need to explain why num_counters_fixed is checked, e.g. the fact that
>> Topdown Slots uses the 4th fixed counter is irrelevant with respect to the legality
>> of setting EBX[7]=1 to hide an unsupported event.
> 
> I was under the impression that the CPUID.0AH:EBX bits were
> independent of the fixed counters. That is, if CPUID.0AH:EAX[31:24] >
> 7 and CPUID.0AH:EBX[7] is clear, then one should be able to program a
> general purpose counter with event selector A4H and umask 01H,
> regardless of whether or not fixed counter 4 exists.

The impression stepped on my toes. This patch is pointless and magnifies my 
clumsiness.

Now the self-contradiction comes to the CLX and older platforms,

	0xA. EAX. Bits 31 - 24: Length of EBX bit vector to enumerate architectural
	performance monitoring events. Architectural event x is supported
	if EBX[x]=0 && EAX[31:24]>x.

however actually for CLX, its CPUID.0xA.EBX[7] = 0 && EAX[31:24] = 0x7 and
we can do exactly the following on a CLX without fixed ctr3

	$ perf stat -e cpu/event=0xa4,umask=0x01/ ls
  	Performance counter stats for 'ls':
          	2,448,185      cpu/event=0xa4,umask=0x01/

So in the context of Intel, we have a performance event that can share the
architectural encoding, but it is not an architectural-defined event.

This is strange, did I miss something somewhere ?

> 
>>
>>                  /*
>>                   * Hide Intel's Topdown Slots architectural event, it's not yet
>>                   * supported by KVM.
>>                   */
>>                  if (eax.split.mask_length > 7)
>>                          cap.events_mask |= BIT_ULL(7);
>>
>>> +                 eax.split.mask_length > 7)
>>> +                     cap.events_mask |= BIT_ULL(7);
>>> +
>>>                edx.split.bit_width_fixed = cap.bit_width_fixed;
>>>                if (cap.version)
>>>                        edx.split.anythread_deprecated = 1;
>>> --
>>> 2.33.1
>>>
diff mbox series

Patch

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 0b920e12bb6d..4fe17a537084 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -782,6 +782,18 @@  static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		eax.split.mask_length = cap.events_mask_len;
 
 		edx.split.num_counters_fixed = min(cap.num_counters_fixed, MAX_FIXED_COUNTERS);
+
+		/*
+		 * The 8th Intel architectural event (Topdown Slots) will be supported
+		 * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.
+		 *
+		 * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
+		 * to make this event unavailable in a consistent way.
+		 */
+		if (edx.split.num_counters_fixed < 4 &&
+		    eax.split.mask_length > 7)
+			cap.events_mask |= BIT_ULL(7);
+
 		edx.split.bit_width_fixed = cap.bit_width_fixed;
 		if (cap.version)
 			edx.split.anythread_deprecated = 1;