On 22 May 2015 at 11:46, Arnd Bergmann [off-list ref] wrote:

On Thursday 21 May 2015 18:01:27 Ard Biesheuvel wrote:

quoted

You could but I wouldn't recommend it since it may also prevent you
from being able to set the boot path, but more importantly, reset and
poweroff may also be available only via UEFI Runtime Services on UEFI
systems.

Right, makes sense. Another option then could be to disable fpsimd
support with preempt-rt on real systems, and document this as a known
source of latency.

Unfortunately, that could result in corruption of userland FP/SIMD
context, since the UEFI Runtime Services are allowed to use those
registers, and only need to adhere to the normal AAPCS rules that
stipulate that q8..q15 are callee-save. That would still result in a
25% latency reduction if we only need to preserve q0..q7 and q16..q31

quoted

So could someone comment on whether virt_efi_set_time() is present in
all the problematic traces? Or was it only chosen because it
illustrates the underlying problem the best? In the former case, there
is an hidden bug that I would like to know about: however, if some
time related facility that is used in a performance (or latency)
sensitive context ultimately ends up programming the wall clock time
in the RTC, then I would expect the same issue to occur on non-UEFI
systems as well.

But without UEFI, updating the RTC would cause much less latency,
because  you don't need to save/restore the fpsimd context, disable
preemption, or call into a potentially unknown external binary
blob, the only latency you'd get there is that of a readl/writel
accessing the RTC register.

Yes, that is right. So the UEFI Runtime Service interface is
disproportionately heavy. But that still doesn't explain why it would
make sense to sync the RTC with the system clock often enough that it
violates maximum latency limits, since normally, you read it on boot
and set it on reset/poweroff.

quoted

One thing I should point out is that this FP/SIMD save/restore is
implemented differently depending on whether it is called from process
context or from hardirq/softirq context. In the former case,
kernel_neon_begin() preserves the userland FP/SIMD context only once,
and only restores it right before returning to userland. This way,
only the first kernel_neon_begin() and the last kernel_neon_end() call
actually induce this latency, and so the average latency could be
quite a bit lower than the worst case (although I understand that few
people may care about the average in an RT context)

Just for my own interest: in what case do we save/restore the fpsimd
state from interrupt context?

For instance, the IEEE802.11 crypto runs in softirq context, but
typically performs a non-trivial amount of crypto work (unless the
hardware takes care of it). Since the accelerated AES-CCM module is
20x faster than C code, it makes sense to stack/unstack 6 NEON
registers and run it on the NEON.