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Michael Ellerman authored
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>
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