Re: [RFC v2 3/5] i386/kvm: Support event with select & umask format in KVM PMU filter

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Feb 06, 2025 at 05:54:32PM +0800, Zhao Liu wrote:
> On Wed, Feb 05, 2025 at 11:07:10AM +0100, Markus Armbruster wrote:
> > Date: Wed, 05 Feb 2025 11:07:10 +0100
> > From: Markus Armbruster <armbru@redhat.com>
> > Subject: Re: [RFC v2 3/5] i386/kvm: Support event with select & umask
> >  format in KVM PMU filter
> > 
> > Zhao Liu <zhao1.liu@intel.com> writes:
> > 
> > > The select&umask is the common way for x86 to identify the PMU event,
> > > so support this way as the "x86-default" format in kvm-pmu-filter
> > > object.
> > 
> > So, format 'raw' lets you specify the PMU event code as a number, wheras
> > 'x86-default' lets you specify it as select and umask, correct?
> 
> Yes!
> 
> > Why do we want both?
> 
> This 2 formats are both wildly used in x86(for example, in perf tool).
> 
> x86 documents usually specify the umask and select fields.
> 
> But raw format could also be applied for ARM since ARM just uses a number
> to encode event.
> 
> > > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > 
> > [...]
> > 
> > > diff --git a/qapi/kvm.json b/qapi/kvm.json
> > > index d51aeeba7cd8..93b869e3f90c 100644
> > > --- a/qapi/kvm.json
> > > +++ b/qapi/kvm.json
> > > @@ -27,11 +27,13 @@
> > >  #
> > >  # @raw: the encoded event code that KVM can directly consume.
> > >  #
> > > +# @x86-default: standard x86 encoding format with select and umask.
> > 
> > Why is this named -default?
> 
> Intel and AMD both use umask+select to encode events, but this format
> doesn't have a name... so I call it `default`, or what about
> "x86-umask-select"?
> 
> > > +#
> > >  # Since 10.0
> > >  ##
> > >  { 'enum': 'KVMPMUEventEncodeFmt',
> > >    'prefix': 'KVM_PMU_EVENT_FMT',
> > > -  'data': ['raw'] }
> > > +  'data': ['raw', 'x86-default'] }
> > >  
> > >  ##
> > >  # @KVMPMURawEvent:
> > > @@ -46,6 +48,25 @@
> > >  { 'struct': 'KVMPMURawEvent',
> > >    'data': { 'code': 'uint64' } }
> > >  
> > > +##
> > > +# @KVMPMUX86DefalutEvent:
> > 
> > Default, I suppose.
> 
> Thanks!
> 
> > > +#
> > > +# x86 PMU event encoding with select and umask.
> > > +# raw_event = ((select & 0xf00UL) << 24) | \
> > > +#              (select) & 0xff) | \
> > > +#              ((umask) & 0xff) << 8)
> > 
> > Sphinx rejects this with "Unexpected indentation."
> > 
> > Is the formula needed here?
> 
> I tried to explain the relationship between raw format and umask+select.
> 
> Emm, where do you think is the right place to put the document like
> this?
> 
> ...
> 
> > > +##
> > > +# @KVMPMUX86DefalutEventVariant:

Typo   s/Defalut/Default/ - repeated many times in this patch.

> > > +#
> > > +# The variant of KVMPMUX86DefalutEvent with the string, rather than
> > > +# the numeric value.
> > > +#
> > > +# @select: x86 PMU event select field.  This field is a 12-bit
> > > +#     unsigned number string.
> > > +#
> > > +# @umask: x86 PMU event umask field. This field is a uint8 string.
> > 
> > Why are these strings?  How are they parsed into numbers?
> 
> In practice, the values associated with PMU events (code for arm, select&
> umask for x86) are often expressed in hexadecimal. Further, from linux
> perf related information (tools/perf/pmu-events/arch/*/*/*.json), x86/
> arm64/riscv/nds32/powerpc all prefer the hexadecimal numbers and only
> s390 uses decimal value.
> 
> Therefore, it is necessary to support hexadecimal in order to honor PMU
> conventions.

IMHO having a data format that matches an arbitrary external tool is not
a goal for QMP. It should be neutral and exclusively use the normal JSON
encoding, ie base-10 decimal. Yes, this means a user/client may have to
convert from hex to dec before sending data over QMP. This is true of
many areas of QMP/QEMU config though and thus normal/expected behaviour.

With regards,
Daniel
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