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Re: [Qemu-devel] [PATCH] hw: add .min_cpus and .default_cpus fields to m

From: Alistair Francis
Subject: Re: [Qemu-devel] [PATCH] hw: add .min_cpus and .default_cpus fields to machine_class
Date: Mon, 6 Nov 2017 15:33:05 -0800

On Mon, Nov 6, 2017 at 3:21 PM, Emilio G. Cota <address@hidden> wrote:
> On Mon, Nov 06, 2017 at 14:32:35 -0800, Alistair Francis wrote:
>> Sorry for the silence here, I noticed these were broken just before I
>> went on holidays but didn't get a chance to fix anything.
>> For the Xilinx case I was thinking of patching the machine code to
>> sanely follow the -smp option.
>> -smp 1 -> Only create 1 A53
>> -smp 4 -> Create 4 A53s
>> -smp 6 -> Create all the CPUs
>> I see a lot of advantages in not forcing the smallest number of CPUs
>> to be 4 unless we really have to.
>> I do see a nice advantage in being able to set the default smp option
>> to something not 1 so the default closely matches hardware, but users
>> can override that if they want to.
>> So for the patch below I like the default_cpus option, but for Xilinx
>> at least I would like to patch the logic to follow the -smp option
>> instead of force a minimum.
> Agreed, honouring -smp would be the right fix.
> Just note that since this is a regression we need the fix to
> be in for 2.11.

Ok, I can spin up a patch for the Xilinx boards in the next day (maybe 2)

> I just took a look at the non-Xilinx boards. It seems simple enough to
> substitute the hard-coded value for smp_cpus, but yet again
> I see "Property" structs that I'm not sure what to do with.
> For instance, bcm2836.c:152:
> static Property bcm2836_props[] = {
>     DEFINE_PROP_UINT32("enabled-cpus", BCM2836State, enabled_cpus, 
> BCM2836_NCPUS),
> };
> What is the purpose here? To enable/disable CPUs with -global args,
> just like it's done for the Xilinx boards? Shouldn't we just use
> -smp for that?


> Also, note that I don't have a way to test these boards, which
> explains why I'm reluctant to change board code. But of
> course if board maintainers step in, I'm all for it :-)

Yeah, I see.

What about if we set default_cpus to the -smp option that is expected.
Then on some of the older boards (like the bcm2836) we print a warning
in the machine init() if the -smp option doesn't match that?

That way the default args work, we allow users to specify a overwrite
but we warn them that it might not work and it ensures all boards
follow a similar flow.

Then if we can test the boards and know that -smp 1 works we can
remove the warning.


> Thanks,
>                 Emilio

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