On 01/14/18 09:52, Konstantin Belousov wrote:
On Sun, Jan 14, 2018 at 09:30:53AM -0800, Nathan Whitehorn wrote:
The immediate consequence of that is that no MI code that knows about
direct maps can possibly take advantage of the direct map on this
platform. Do we really want that to save some conditional logic that
would get optimized out on amd64 and arm64 anyway? I really do not see
the benefit here.
It is not clear what do you mean. Are you saying that there is no benefit
of providing the conditional logic, or that it is not benefit of exclusing
powerpc ?
Sorry, that was poorly stated. Let me try again:
If we make a PPC_PHYS_TO_DMAP(), but there is an MI PHYS_TO_DMAP() API,
consumer code in the MI parts of the kernel won't be able to benefit
from the PPC direct map, which seems unfortunate. The cost from a code
perspective of having an if (direct_map_available) seems low, since on
systems where direct_map_available is defined to be 1, the compiler will
optimize it to the same code as if gated by #ifdef. It might be more
cumbersome to write the code, however.
I do not object against adding the conditional, but it should not be
too clumsy to use.
OK. Let me try to draft something in the next couple days and see how
much of a pain it is in practice.
-Nathan
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