Re: [PATCH v3 0/6] Static calls

[H. Peter Anvin]

On 1/10/19 9:31 AM, Linus Torvalds wrote:
> On Wed, Jan 9, 2019 at 2:59 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>>
>> NOTE: At least experimentally, the call destination writes seem to be
>> atomic with respect to instruction fetching.  On Nehalem I can easily
>> trigger crashes when writing a call destination across cachelines while
>> reading the instruction on other CPU; but I get no such crashes when
>> respecting cacheline boundaries.
> 
> I still doubt ifetch is atomic on a cacheline boundary for the simple
> reason that the bus between the IU and the L1 I$ is narrower in older
> CPU's.
> 

As far as I understand, on P6+ (and possibly earlier, but I don't know) it is
atomic on a 16-byte fetch datum, at least for Intel CPUs.

However, single byte accesses are always going to be safe.

	-hpa