linux-kernel - Re: [DROP PATCH v2] KVM: x86/pmu: Make top-down.slots event unavailable in supported leaf

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Date:   Fri, 7 Jan 2022 12:13:29 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [DROP PATCH v2] KVM: x86/pmu: Make top-down.slots event
 unavailable in supported leaf

On 7/1/2022 4:12 am, Jim Mattson wrote:
> On Thu, Jan 6, 2022 at 10:09 AM Sean Christopherson <seanjc@...gle.com> wrote:
>>
>> On Thu, Jan 06, 2022, Like Xu wrote:
>>> From: Like Xu <likexu@...cent.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@...cent.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
>>>