On Mon, Dec 12, 2022 at 01:03:52PM +0100, Morten Brørup wrote:
> > From: Bruce Richardson [mailto:bruce.richard...@intel.com]
> > Sent: Monday, 12 December 2022 12.21
> >
> > On Mon, Dec 12, 2022 at 12:02:32PM +0100, Morten Brørup wrote:
> > > > From: Bruce Richardson [mailto:bruce.richard...@intel.com]
> > > > Sent: Monday, 12 December 2022 11.32
> > > >
> > > > On Mon, Dec 12, 2022 at 02:42:55PM +0800, Huisong Li wrote:
> > > > > Some lib telemetry interfaces add the 'u32' and 'u64' data by the
> > > > > rte_tel_data_add_dict/array_int API. This may cause data
> > conversion
> > > > error
> > > > > or data truncation.
> > > > >
> > > > > The 'u32' data can not be assigned to signed 32-bit integer.
> > However,
> > > > > assigning to u64 is very wasteful, after all, the buffer capacity
> > of
> > > > each
> > > > > transfer is limited. So it is necessary for 'u32' data to add
> > usigned
> > > > > 32-bit integer type and a series of 'u32' operation APIs.
> > > > >
> > > > > This patchset uses the new 'u32' API to resolve the problem of
> > data
> > > > > conversion error, and use the 'u64' API to add 'u64' data.
> > > > >
> > > > > In addition, this patchset introduces two APIs to store u32 and
> > u64
> > > > > values as hexadecimal encoded strings in telemetry library.
> > > > >
> > > > > --- -v3: fix a misspelling mistake in commit log. -v2: - fix ABI
> > > > break
> > > > > warning. - introduce two APIs to store u32 and u64 values as
> > > > hexadecimal
> > > > > encoded strings.
> > > > >
> > > > I'm not convinced about adding the u32 value generically to the
> > > > telemetry
> > > > lib - except in the case of having explicit function calls for u32
> > vs
> > > > u64
> > > > hex strings. Having a u32 type doesn't gain us any space internally
> > > > over a
> > > > u64 value, since all values are in a union type. Also, for output
> > as
> > > > json,
> > > > the numeric values are all output as decimal values, meaning that
> > the
> > > > value
> > > > 1 appears as the same size in the output string whether it is a u32
> > or
> > > > u64
> > > > type. Now, it may save space in a future binary output format, but
> > even
> > > > then it still may not do so.
> > >
> > > I agree that a u32 doesn't gain any space internally.
> > >
> > > However, many SNMP counters are unsigned 32 bit, and expected to wrap
> > around as such.
> > >
> > > So I suppose the u32 type might be useful for SNMP, if obtained
> > through the telemetry library.
> > >
> > > Alternatively, we could somehow reuse the u64 type and require the
> > application to pass (value & UINT32_MAX) to the u64 functions. To make
> > this easy to use, we should add some wrappers to do it for the
> > application. And eventually we would probably end up with something
> > very similar to this patch.
> > >
> >
> > I think just using the u64 functions is probably simplest and best
> > right
> > now. If we add support for something like snmp then yes, it would make
> > sense to explicitly add it, but it seems like a lot of extra code for
> > little or no benefit until we support something like that.
>
> <rant>
> If we wanted to fix this generally, we should rely on type promotion, so the
> existing _int function should be updated to take an int64_t value, and the
> _u64 function should be renamed to _uint (and still take an uint64_t value).
> However, that would break the ABI, and would need to go through some process
> for that. So let's not change this now.
> </rant>
Yes, not making "int" an "i64" type was a poor design decision on my part
in the earlier versions. Thankfully negative values are rarely needed
beyond the range of 32-bits, but we should probably look to update this as
you suggest at the next ABI break window.
>
> I tend to agree with Bruce on this: Let's get rid of the new u32 functions,
> and rely on the u64 functions for that instead.
>
> >
> > > >
> > > > Therefore, I'd tend to keep the existing u64 type as-is, and
> > instead
> > > > only
> > > > add the functions for outputting hex values. Those hex output
> > functions
> > > > could take an additional parameter indicating the desired hex
> > output
> > > > length, as there could well be cases where we want just 16-bit hex
> > > > value
> > > > too.
> > >
> > > The way I read the patch series, the hex output length is not fixed,
> > but an u64 value of 5 will be output as 0x5, not 0x0000000000000005.
> > >
> > > So the only benefit of having both u32_hex and u64_hex functions is
> > to avoid type promotion from uint32_t to uint64_t on input for 32 bit
> > values.
> > >
> > > Instead of passing a 3rd parameter or adding u16_hex functions, we
> > could consider just having one set of hex functions using uint64_t for
> > the value, and rely on type promotion for 32 and 16 bit values.
> > >
> >
> > +1 to having only a single hex function, and I think type promotion
> > should
> > work fine.
> >
> > However, I still think it might be worthwhile allowing the user to pass
> > in
> > a min output length parameter too. I find for many hex strings having
> > the
> > leading zeros to explicitly show the length can be useful. The value
> > "0"
> > could cover the current behaviour of no-padding, otherwise the
> > parameter
> > should indicate the number of bits to be displayed. (If we want to lock
> > it
> > down we can use an enum parameter rather than int to limit it to 0, 8,
> > 16,
> > 32 or 64 bit displayed values).
>
> An extra parameter for minimum output length (in number of input bits) would
> be very nice, and makes avoids a set of functions for each bit width.
>
> (I don't think it should be lock it down to some special bit lengths; there
> is no need to prevent 24 bit or other exotic bit widths.)
>
> Something like this:
>
> char str[64]; // Big enough length.
> if (bits != 0) {
> char format[16]; // Big enough length.
> sprintf(format, "0x0%u%" PRIx64, (bits + 3) / 4);
> sprintf(str, format, value);
> } else {
> sprintf(str, "0x%" PRIx64, value);
> }
>
LGTM.
