Re: [Xen-devel] [RFC][PATCH] 1/3] [XEN] Use explicit bit sized fieldsfor exported xentrace data.

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> The ultimate best way of doing it would be to have trace functions that
> took a format string and a variable number of arguments.

Agreed.

> The actual 
> trace record written in the buffer would just contain the record type
> and the length of the record, followed by the variable data. The format
> string would be written out in an a separate segment, enabling it to be
> extracted and used by the trace post-process tool to pretty print the
> records.

Cool.

It would be fairly trivial to declare a name for each "type" and allow 
userspace to query the type name <-> type ID mapping.  This then removes all 
responsibility for determining output formatting from userspace, and should 
make everything neater.

Nice.  All we need is a coder - anyone interested?

Cheers,
Mark

> Ian
>
> > The second case involves an extra copy, but that shouldn't be a big
> > deal.  It has the advantage of being self-contained, and the trace
> > code can make the record "wrap around" transparently.
> >
> > The first means no copying, but it also means no "wrap around"; if
> > there's not enough room at the end of a buffer, the space would just
> > have to be left empty.  That's not probably such a big deal, though.
> > The bigger problem comes if several "open" trace records happen at
> > once.  It's technically possible that the trace buffer will wrap
> > around before a function is done writing to its original buffer.
> >
> > In both of these cases, the common "TRACE_nD" macros can be left, I
> > think.  We might want to add "TRACE_nDL" for 64-bit values, and then
> > let those who need more flexible trace structures call trace_var()
> > directly.
> >
> > This way of doing things also has the advantage that the trace record
> > can be defined in a public header somewhere, and used by user-space
> > analysis tools as well as the hypervisor tracing code.
> >
> > Thoughts?
> >
> >  -George
> >
> > On 12/5/06, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:
> > > > There's no reason not to make the trace format more flexible.
>
> There's a
>
> > > > question about how you represent trace points in the Xen code
>
> though,
>
> > when
> >
> > > > the format is no longer a list of fixed size integers.
> > >
> > > I can see two main possibilities.  One involving a variadic function
>
> and
>
> > one
> >
> > > involving mega macros of doom.
> > >
> > > One possibility would be a trace() function taking a variable number
>
> of
>
> > > arguments, i.e.
> > >
> > > void trace(type, unsigned char data1, unsigned char data2, ... etc)
> > >
> > > And a set of arch-defined macros (or at least bitness / endian
>
> defined
>
> > > macros).  Eg. on x86 we might have:
> > >
> > > #define TRACE_U16(d) ((unsigned char)(d & 255)), ((unsigned char)(d
> >
> > 8))
> >
> > > We'd need to verify whether the extra processing had a measurable
> >
> > performance
> >
> > > impact, however.
> > >
> > > Another alternative would be to make the array of trace buffers
>
> globally
>
> > > accessible and then use a set of macros for the trace() instead of
>
> an
>
> > inline
> >
> > > function.  The macros could then look something like (pseudocode):
> > >
> > > struct trace_record {
> > > u32 type;
> > > u32 data_len;
> > > char data[]
> > > };
> > >
> > > char *trace_buffer[NR_CPUS]
> > >
> > > #define open_trace(type)      \
> > >                               do { \
> > >                                   disable local irqs
> >
> > \
> >
> > >                                   struct trace_record *record =
> >
> > \
> >
> > &trace_buffer[cpu][producer_idx]; \
> >
> > >                                   record->type = (u32)type
> >
> > \
> >
> > >                                   record->data_len = 0;
> > >
> > > #define trace_u16(data)       *(u16 *)record->data[record->data_len]
>
> =
>
> > data \
> >
> > >                               record->data_len += sizeof(u16)
> > >
> > > ... etc for different data types, with appropriate variations if
> >
> > necessary for
> >
> > > different platforms ...
> > >
> > > #define close_trace()       \
> > >                             inc producer counter by sizeof(struct
> >
> > \
> >
> > >                             trace_record) + record->data_len for
> >
> > userspace \
> >
> > >                             to see \
> > >                             re-enable local irqs
> >
> > \
> >
> > >                             } while(0)
> > >
> > >
> > > Things become unhappy here because there'd need to be some kind of
>
> bounds
>
> > > checking in here to determine whether we need to wrap to the
>
> beginning of
>
> > the
> >
> > > trace buffer again.  The alternatives as I see them would be either:
> > >
> > > a) include code in each data macro to check if we'd reached the end
>
> of
>
> > the
> >
> > > buffer and wrap the data appropriately, or
> > > b) include code that'll simply copy everything we've built so far to
>
> the
>
> > > beginning of the trace buffer and start again.
> > >
> > > Either way is going to be ugly and unpleasant.  Also, we have the
>
> problem
>
> > of
> >
> > > not knowing whether we're going to wrap OR run out of space until
>
> we're
>
> > part
> >
> > > way through the trace record, although in this instance, I guess we
>
> could
>
> > > just change to create a "missed data" record.
> > >
> > > I think the first approach (variadic function) above is probably
>
> nicer.
>
> > We
> >
> > > can always make a few macros to make common cases (e.g. recording a
>
> type
>
> > and
> >
> > > a single u64 of data) less verbose.
> > >
> > > Any thoughts?
> > >
> > > Cheers,
> > > Mark
> > >
> > > --
> > > Dave: Just a question. What use is a unicyle with no seat?  And no
> >
> > pedals!
> >
> > > Mark: To answer a question with a question: What use is a
>
> skateboard?
>
> > > Dave: Skateboards have wheels.
> > > Mark: My wheel has a wheel!
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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