Re: [PATCH 2/3] x86_64: Define 128-bit memory-mapped I/O operations

[David Miller <davem@davemloft.net>]

From: Ben Hutchings <bhutchings@solarflare.com>
Date: Wed, 22 Aug 2012 03:04:11 +0100

> On Tue, 2012-08-21 at 18:37 -0700, H. Peter Anvin wrote:
>> On 08/21/2012 06:23 PM, Ben Hutchings wrote:
>> > Define reado(), writeo() and their raw counterparts using SSE.
>> > 
>> > Based on work by Stuart Hodgson <smhodgson@solarflare.com>.
>> 
>> It would be vastly better if we explicitly controlled this with
>> kernel_fpu_begin()/kernel_fpu_end() rather than hiding it in primitives
>> than might tempt the user to do very much the wrong thing.
>> 
>> Also, it needs to be extremely clear to the user that these operations
>> use the FPU, and all the requirements there need to be met, including
>> not using them at interrupt time.
> 
> Well we can sometimes use the FPU state at IRQ time, can't we
> (irq_fpu_usable())?   So we might need, say, try_reado() and
> try_writeo() with callers expected to fall back to alternatives.  (Which
> they must have anyway for any architecture that doesn't support this.)

I really hope we eventually get rid of this rediculous restriction the
x86 code has.

It really needs a proper stack of FPU state saves like sparc64 has.

Half of the code and complexity in arch/x86/crypto/ would just
disappear, because most of it has to do with handling this obtuse
FPU usage restriction which shouldn't even be an issue in the first
place.

I continually see more and more code that has to check this
irq_fpu_usable() thing, and have ugly fallback code, and therefore is
a sign that this really needs to be fixed properly.