Re: [1/4] powerpc/kernel: Drop HMT_MEDIUM_PPR_DISCARD

[Michael Ellerman <mpe@ellerman.id.au>]

On Wed, 2015-25-11 at 03:25:16 UTC, Michael Ellerman wrote:
> HMT_MEDIUM_PPR_DISCARD is a macro which is present at the start of most
> of our first level exception handlers. It conditionally executes a
> HMT_MEDIUM instruction, which sets the processor priority to medium.
> 
> On on modern systems, ie. Power7 and later, it is nop'ed out at boot.
> All it does is make the exception vectors more cramped, and consume 4
> bytes of icache.
> 
> On old systems it has the effect of boosting the processor priority at
> the start of exception processing. If we were previously in the idle
> loop for example, we may be at low or very low priority. This is
> desirable as we want to process the exception as fast as possible.
> 
> However looking closely at the generated code, we see that in all cases
> we execute another HMT_MEDIUM just four instructions later. With code
> patching applied, the final code on an old (Power6) system will look
> like, eg:
> 
>   c000000000000300 <data_access_pSeries>:
>   c000000000000300:	7c 42 13 78	mr	r2,r2		<-
>   c000000000000304:	7d b2 43 a6	mtsprg	2,r13
>   c000000000000308:	7d b1 42 a6	mfsprg	r13,1
>   c00000000000030c:	f9 2d 00 80	std	r9,128(r13)
>   c000000000000310:	60 00 00 00	nop
>   c000000000000314:	7c 42 13 78	mr	r2,r2		<-
> 
> So I suggest that the added code complexity of HMT_MEDIUM_PPR_DISCARD is
> not justified by the benefit of boosting the processor priority for the
> duration of four instructions, and therefore we drop it.
> 
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Series applied to powerpc next.

https://git.kernel.org/powerpc/c/d6265aeaf815801ad53a95f11c

cheers