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Re: [Xen-devel] [RFC][PATCH] 1/3] [XEN] Use explicit bit sized fields fo

To: Keir Fraser <keir@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [RFC][PATCH] 1/3] [XEN] Use explicit bit sized fields for exported xentrace data.
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 5 Dec 2006 16:54:52 +0000
Cc: George Dunlap <dunlapg@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Tony Breeds <tony@xxxxxxxxxxxxxxxxxx>
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> 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?


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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