r */
(2)  xor    %esi,%esi				/* no error code */
(5)  callq  ffffffff80213446 <do_overflow>
(5)  jmpq   ffffffff8030e460 <error_exit>

And one for an exception with errorcode like this:

<segment_not_present>:
(6)  callq  *0x1cab92(%rip)        # ffffffff803dd448 <pv_irq_ops+0x38>
(4)  sub    $0x78,%rsp				/* space for registers */
(5)  callq  ffffffff8030e3b0 <error_entry>
(3)  mov    %rsp,%rdi				/* pt_regs pointer */
(5)  mov    0x78(%rsp),%rsi			/* load error code */
(9)  movq   $0xffffffffffffffff,0x78(%rsp)	/* no syscall */
(5)  callq  ffffffff80213209 <do_segment_not_present>
(5)  jmpq   ffffffff8030e460 <error_exit>

Unfortunately, this last type is more than 32 bytes. But the total space
savings due to this patch is about 2500 bytes on an smp-configuration,
and I think the code is clearer than it was before. The tested kernels
were non-paravirt ones (i.e., without the indirect call at the top of
the exception handlers).

Anyhow, I tested this patch on top of a recent -tip. The machine
was an 2x4-core Xeon at 2333MHz. Measured where the delays between
(almost-)adjacent rdtsc instructions. The graphs show how much
time is spent outside of the program as a function of the measured
delay. The area under the graph represents the total time spent
outside the program. Eight instances of the rdtsctest were
started, each pinned to a single cpu. The histogams are added.
For each kernel two measurements were done: one in mostly idle
condition, the other while running "bonnie++ -f", bound to cpu 0.
Each measurement took 40 minutes runtime. See the attached graphs
for the results. The graphs overlap almost everywhere, but there
are small differences.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
†ZŒª“