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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.02.2020 11:03, Roger Pau Monné wrote:
> 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?

Depends on how many CPUs there are, i.e. how long the loop otherwise
would be. When xmalloc() doesn't need to turn to the page allocator,
I think it won't be overly slow. Another option would be to avoid
coalescing in a slightly different way (without having to fiddle
with the scratch mask) when called in interrupt context.

Jan

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