Re: [Qemu-trivial] [Qemu-devel] [PATCH] vl: remove (max_cpus > 255) check from smp_parse

[Andreas Färber]

Am 02.12.2013 18:06, schrieb Michael Tokarev:
> 25.11.2013 07:39, Alexey Kardashevskiy wrote:
>> Since modern POWER7/POWER8 chips can have more that 256 CPU threads
>> (>2000 actually), remove this check from smp_parse.
>>
>> The CPUs number is still checked against machine->max_cpus and this check
>> should be enough not to break other archs.

"should be" is not exactly the highest level of confidence for a
"trivial" patch... :/

> []
>> -    if (max_cpus > 255) {
>> -        fprintf(stderr, "Unsupported number of maxcpus\n");
>> -        exit(1);
>> -    }

I believe Eduardo touched that code last for NUMA, so let's CC him.

> I don't know whenever this is actually safe.  Do we have any static arrays
> of size 255 somewhere, which will be overflowed without this check? :)

s390 has the ipi_states[] array, but not fixed to that size.

x86 APIC IDs I think have or had a limitation to 255 rather than 16-bit?
Igor?

Alexey, did you actually check that, e.g., x86 machines don't break with
256 or 257 CPUs now?

Adding a qtest would be one way to prove that at least the QEMU code of
all other architectures doesn't break with the ppc change.

Regards,
Andreas

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