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Re: [Qemu-devel] [PATCH] pc: q35: bump max_cpus to INT32_MAX

From: Radim Krčmář
Subject: Re: [Qemu-devel] [PATCH] pc: q35: bump max_cpus to INT32_MAX
Date: Wed, 22 Mar 2017 17:54:39 +0100

2017-03-22 18:06+0200, Michael S. Tsirkin:
> On Wed, Mar 22, 2017 at 04:59:06PM +0100, Radim Krčmář wrote:
>> QEMU does not allocate based on machine's max_cpus, but only uses it to
>> limit the maximum selected by user and the actual limit of VCPUs is
>> enfoced by other components:
>>  - machine without vIOMMU ends at 255 VCPUs
>>  - TCG currently doesn't support x2APIC, so it also ends below 256
>>  - KVM with vIOMMU cannot exceed the KVM limit (currently 288)
>> Using a big value should bring no drawbacks and the benefit is that
>> components (especially out-of-tree KVM) can bump their limits without
>> touching machine's max_cpus.
>> max_cpus is unsigned, but it is treated as signed at least in printf and
>> the other two billion VCPU won't be needed for a while, so we can ignore
>> possible bugs by using signed max.
>> Signed-off-by: Radim Krčmář <address@hidden>
>> ---
>>   Should the 2.9 machine type still have 288?
> It doesn't look like a bugfix to me.
> So if we can defer past 2.9 that would be best IMO.

Sure, I'll wait until the 2.10 machine type pops up.


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