Re: [Xen-devel] [PATCH] libxl: Make 'xl vcpu-set' work properly on overcommited hosts.

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, May 7, 2013 at 9:40 PM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> The libxl_cpu_bitmap_alloc(..) function, if provided with a zero
> value for max CPUs will call xc_get_max_cpus() which will retrieve
> the number of physical CPUs the host has. This is usually
> OK if the guest's maxvcpus <= host pcpus. But if the value
> is different, then the bitmap for VCPUs is limited by the
> number of CPUs the host has.
>
> This is incorrect as what we want is to hotplug in the guest
> the amount of CPUs that the user specified on the command line
> and not be limited by the amount of physical CPUs.
>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

Given the cost/benefits of this, I'm inclined to say this should wait
until the 4.4 window opens.  Having more guest vcpus than host pcpus
is not an urgent need, and there is a chance (however small) of this
exposing some other kind of bug.

 -George

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