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Re: [Xen-devel] [PATCH 2/3] x86/smp: use a dedicated scratch cpumask in send_IPI_mask

On Thu, Feb 13, 2020 at 10:59:29AM +0100, Jan Beulich wrote:
> On 12.02.2020 17:49, Roger Pau Monne wrote:
> > Using scratch_cpumask in send_IPI_mak is not safe because it can be
> > called from interrupt context, and hence Xen would have to make sure
> > all the users of the scratch cpumask disable interrupts while using
> > it.
> > 
> > Instead introduce a new cpumask to be used by send_IPI_mask, and
> > disable interrupts while using.
> My first thought here was: What about NMI or #MC context? Even if
> not using the function today (which I didn't check), there shouldn't
> be a latent issue introduced here preventing them from doing so in
> the future. Instead I think you want to allocate the scratch mask
> dynamically (at least if caller context doesn't allow use of the
> generic one), and simply refrain from coalescing IPIs if this
> allocations fails.

Hm, isn't this going to be quite expensive, and hence render the
benefit of using the shorthand moot?

Thanks, Roger.

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