Re: [perfmon2] comments on Performance Counters for Linux (PCL)

[stephane eranian <eranian@googlemail.com>]

On Fri, May 29, 2009 at 12:50 PM, stephane eranian
<eranian@googlemail.com> wrote:
> Hi,
>
> On Fri, May 29, 2009 at 1:24 AM, Paul Mackerras <paulus@samba.org> wrote:
>> Ingo Molnar writes:
>>
>>> * Corey Ashford <cjashfor@linux.vnet.ibm.com> wrote:
>>> >> So you're suggesting to artificually strech periods by say
>>> >> composing a single overflow from smaller ones, ignoring the
>>> >> intermediate overflow events?
>>> >>
>>> >> That sounds doable, again, patch welcome.
>>> >
>>> > I definitely agree with Stephane's point on this one.  I had
>>> > assumed that long irq_periods (longer than the width of the
>>> > counter) would be synthesized as you suggest.  If this is not the
>>> > case, PCL should be changed so that it does, -or- at a minimum,
>>> > the user should get an error back stating that the period is too
>>> > long for the hardware counter.
>>>
>>> this looks somewhat academic - at least on x86, even the fastest
>>> events (say cycles) with a 32 bit overflow means one event per
>>> second on 4GB. That's not a significant event count in practice.
>>> What's the minimum width we are talking about on Power?
>>
>> 32 bits, but since the top bit is effectively a level-sensitive
>> interrupt request, the maximum period in hardware is 2^31 counts.
>>
> This is exactly the same on Intel X86: 31 bits.
> AMD is different.
>
> Unfortunately, the Intel document is very obscure about this restriction.
>
> Let's take Core. 40-bit wide counters. If you read with RDPMC
> you get 40-bit worth of data, same with RDMSR.
>
> If you write with WRMSR, you can only modify the bottom 32
> bits. But bit 31 is the sign bit. Bit 32-39 are sign extension
> of bit 31.
>
Let me rephrase this a bit. You can actually modify all bits
with WRMSR. but the sign extension determines the value
of bits 32-39 based on bit 31.

Forgot to mention that since Penryn, if you try to set bits 40-63
with WRMSR, you get a fault.