Re: [PATCH 01/22] x86: add methods for writing of an MSR on several CPUs

["H. Peter Anvin" <hpa@zytor.com>]

Borislav Petkov wrote:
> 
> We currently need them for enabling the NB error reporting bank over
> MCG_CTL on each core on the node. The question is whether we really need
> the concurrency when accessing an MSR on several cores. With MCG_CTL,
> BKDG says "It is expected that this register is programmed to the same
> value in all nodes," but nothing concerning concurrency.
> 
> But you're right, if this interface is supposed to be generic enough,
> it is probably wise to access an MSR concurrently. I could imagine
> an obscure case where this is required. However, is sending IPIs
> (smp_call_function_many) guaranteeing the needed concurrency? Or, should
> it be more like how the mtrr code jumps through hoops (set_mtrr())
> in order to ensure that _ALL_ registers have been written _before_
> continuing?
> 

smp_call_function_many() does allow concurrent execution.  Looping over
a list with smp_call_function_one() -- which you currently have -- is
serializing, at which point we might just push the loop into the caller
rather than worrying about a new interface.

	-hpa