> On 26 Jul 2018, at 14:15, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote: > >> On 2018-07-26 11:15:52 [+0200], Ard Biesheuvel wrote: >> On 26 July 2018 at 11:04, Sebastian Andrzej Siewior >> <bigeasy@xxxxxxxxxxxxx> wrote: >>> Based on measurements the EFI functions get_variable / >>> get_next_variable take up to 2us. The functions get_time, set_time take >>> around 10ms. Those 10ms are too much. Even one ms would be too much. >> >> You cannot extrapolate from these numbers. If the lack of worst case >> guarantees on an EFI system is a problem for -rt, the only meaningful >> course of action is to disable EFI runtime services entirely. > > Oh boy. I disable EFI entirely because then the bootloader won't boot > due to missing efi-stub. But I could disable the runtime-wrappers if > that is what you meant. Patch below. > What i mean is whatever you end up with if you pass efi=noruntime to the kernel. But as i mentioned before, you may also lose the ability to reboot/shut down your system this way. Can you please double check? >> I think SetVariable() may be even worse than GetTime(), given that NOR >> flash updates may involve erasing blocks. > > Here we go. "Read" can be triggered from userland via sysfs / efivars. > "Write" is (currently) limited to pstore? > No you can also write to variables via efivarfs and sysfs. >>> The funny part is that EFI is invoked with enabled interrupts. >>> According to my tracing I see the interrupt almost 10ms later which >>> indicates that EFI is disabling interrupts while doing its thing. >> >> Yes, and this is also highly implementation specific. Basing this kind >> of policy on a single implementation is not very wise imo. > > even if interrupts were not disabled then there would be no context > switch on exit from interrupt path due to the preempt-disable part. So > no improvement. > >>> This was observed on "EFI v2.60 by SoftIron Overdrive 1000". I don't say >>> that every EFI implementation does this but given that it has to access a >>> slow bus like I2C/SPI it is expected. >>> >>> Disable EFI's RTC driver on RT. >>> >> >> Other calls to GetTime() would still be permitted in this case, so >> this seems like a partial solution (although no other calls seem to >> exist atm) > > I *assumed* pstore would use the SetVariable during warning/bug. The > pstore thingy might be useful. Not sure. > > ------ >8 > > Subject: [PATCH RT] arm64: Disable EFI runtime services on RT > > Based on meassurements the EFI functions get_variable / > get_next_variable take up to 2us which looks okay. > The functions get_time, set_time take around 10ms. Those 10ms are too > much. Even one ms would be too much. > Ard mentioned that SetVariable might even trigger larger latencies if > the firware will erase flash blocks on NOR. > > The time-functions are used by efi-rtc and can be triggered during > runtimed (either via explicit read/write or ntp sync). > > The variable write could be used by pstore. > These functions can be disabled without much of a loss. The poweroff / > reboot hooks will be provided by PSCI. > > Disable EFI's runtime wrappers. > > This was observed on "EFI v2.60 by SoftIron Overdrive 1000". > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > --- > arch/arm64/Kconfig | 2 +- > drivers/firmware/efi/arm-runtime.c | 2 ++ > 2 files changed, 3 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index 111db4e44fcf..68adcb0f4de6 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -1250,7 +1250,7 @@ config EFI > select LIBFDT > select UCS2_STRING > select EFI_PARAMS_FROM_FDT > - select EFI_RUNTIME_WRAPPERS > + select EFI_RUNTIME_WRAPPERS if !PREEMPT_RT_BASE > select EFI_STUB > select EFI_ARMSTUB > default y > diff --git a/drivers/firmware/efi/arm-runtime.c b/drivers/firmware/efi/arm-runtime.c > index 5889cbea60b8..3e96db10c034 100644 > --- a/drivers/firmware/efi/arm-runtime.c > +++ b/drivers/firmware/efi/arm-runtime.c > @@ -140,8 +140,10 @@ static int __init arm_enable_runtime_services(void) > } > > /* Set up runtime services function pointers */ > +#ifdef CONFIG_EFI_RUNTIME_WRAPPERS > efi_native_runtime_setup(); > set_bit(EFI_RUNTIME_SERVICES, &efi.flags); > +#endif > > return 0; > } > -- > 2.18.0 > -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
