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Re: [PATCH v2 2/4] smbus: Fix spd_data_generate() error API violation

From: BALATON Zoltan
Subject: Re: [PATCH v2 2/4] smbus: Fix spd_data_generate() error API violation
Date: Wed, 22 Apr 2020 21:32:03 +0200 (CEST)
User-agent: Alpine 2.22 (BSF 395 2020-01-19)

On Wed, 22 Apr 2020, Philippe Mathieu-Daudé wrote:
On 4/22/20 4:27 PM, BALATON Zoltan wrote:
On Wed, 22 Apr 2020, Markus Armbruster wrote:
The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second

spd_data_generate() can pass @errp to error_setg() more than once when
it adjusts both memory size and type.  Harmless, because no caller
passes anything that needs adjusting.  Until the previous commit,
sam460ex passed types that needed adjusting, but not sizes.

spd_data_generate()'s contract is rather awkward:

   If everything's fine, return non-null and don't set an error.

   Else, if memory size or type need adjusting, return non-null and
   set an error describing the adjustment.

   Else, return null and set an error reporting why no data can be

Its callers treat the error as a warning even when null is returned.
They don't create the "smbus-eeprom" device then.  Suspicious.

Since the previous commit, only "everything's fine" can actually
happen.  Drop the unused code and simplify the callers.  This gets rid
of the error API violation.

This leaves board code no chance to recover from values given by user that won't fit without duplicating checks that this function does. Also this will abort without giving meaningful errors if an invalid value does get through and result in a crash which is not used friendly. So I don't like this but if others think this is acceptable maybe at least unit test should be adjusted to make sure aborts cannot be triggered by user for values that are not usually tested during development.

Agreed. Do you have an example (or more) to better show Markus this code use? So we can add tests.

After Markus's patches probably nothing uses it any more but this comes with the result that previously giving some random value such as -m 100 did produce a working sam460ex machine after some warnings but now it just thows back some errors to the user which may or may not be helpful to them.

Personally I'd use a script to generate a dumb static array of all possible sizes...

Maybe testing with the biggest valid value such as -m 2048 (that's commonly used probably) and an invalid value such as -m 100 might be enough. Testing all possible values might take too long and would not test what happens with invalid values. Ideally those invalud values should also work like before a0258e4afa but should at least give a meaningful warning so the user can fix the command line without too much head scratching. Actually that commit was from Igor not from Marcus so sorry for attributing that to Marcus too, I remembered wrong.

By the way you could argue that on real machine you cannot plug certain combinations of memory modules so it's enough to model that but I think QEMU does not have to be that strict and also support configs that cannot happen on real hadware but would work. This might be useful for example if you have some ammount of memory to set aside for a VM on a host but that's not a size that exists in memory modules on real hardware. This also works on pc machine in qemu-system-i386 for example: it accepts -m 100 and does its best to create a machine with such unrealistic size. The sam460ex did the same (within SoC's limits) and before a0258e4afa -m 100 was fixed up to 96 MB which is now not possible due to change in QEMU internal APIs. This probably isn't important enough to worth the extra effort to support but would have been nice to preserve.


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