Re: [Xen-devel] [PATCH] x86/vPMU: don't blindly assume IA32_PERF_CAPABILITIES MSR exists

["Chen, Farrah" <farrah.chen@xxxxxxxxx>]

I applied this patch to Xen and retested, Xen on KVM booted up successfully, 
thanks a lot.

Thanks,
Fan

-----Original Message-----
From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
Sent: Wednesday, February 26, 2020 6:56 PM
To: Jan Beulich <jbeulich@xxxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Chen, Farrah <farrah.chen@xxxxxxxxx>; Hao, 
Xudong <xudong.hao@xxxxxxxxx>; Gao, Chao <chao.gao@xxxxxxxxx>; Wei Liu 
<wl@xxxxxxx>
Subject: Re: [PATCH] x86/vPMU: don't blindly assume IA32_PERF_CAPABILITIES MSR 
exists

On 26/02/2020 10:39, Jan Beulich wrote:
> On 26.02.2020 11:09, Roger Pau Monné wrote:
>> On Wed, Feb 26, 2020 at 10:19:19AM +0100, Jan Beulich wrote:
>>> Just like VMX'es lbr_tsx_fixup_check() the respective CPUID bit 
>>> should be consulted first.
>>>
>>> Reported-by: Farrah Chen <farrah.chen@xxxxxxxxx>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> --- a/xen/arch/x86/cpu/vpmu_intel.c
>>> +++ b/xen/arch/x86/cpu/vpmu_intel.c
>>> @@ -900,7 +900,6 @@ int vmx_vpmu_initialise(struct vcpu *v)
>>>  
>>>  int __init core2_vpmu_init(void)
>>>  {
>>> -    u64 caps;
>>>      unsigned int version = 0;
>>>      unsigned int i;
>>>  
>>> @@ -932,8 +931,14 @@ int __init core2_vpmu_init(void)
>>>  
>>>      arch_pmc_cnt = core2_get_arch_pmc_count();
>>>      fixed_pmc_cnt = core2_get_fixed_pmc_count();
>>> -    rdmsrl(MSR_IA32_PERF_CAPABILITIES, caps);
>>> -    full_width_write = (caps >> 13) & 1;
>>> +
>>> +    if ( cpu_has_pdcm )
>>> +    {
>>> +        uint64_t caps;
>>> +
>>> +        rdmsrl(MSR_IA32_PERF_CAPABILITIES, caps);
>>> +        full_width_write = (caps >> 13) & 1;
>> Will PMU work without PDCM?

The performance counter interface in CPUs predate the introduction of PERF_CAPS.

>> I've been grepping the Intel SDMs, but the only mention is that PDCM 
>> signal the availability of MSR_IA32_PERF_CAPABILITIES.
> Well, there's no other use of the MSR afaics except for getting the 
> one bit here, so I assume it'll work.

It is an off-by-default, outside security support area of functionality with 
known functional bugs outstanding against it.

"not crash" is a fine improvement on the status quo.

~Andrew
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