> 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>
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);
}
>
> All that said, I'm not massively concerned if we want to just keep the
> current approach of always just printing without any leading zeros.
> It's a
> nice-to-have only for me.
>
> /Bruce
