On 3 February 2016 at 10:43, Ingo Molnar <mi...@kernel.org> wrote: > > * Matt Fleming <m...@codeblueprint.co.uk> wrote: > >> From: Ard Biesheuvel <ard.biesheu...@linaro.org> >> >> The UEFI spec allows Runtime Services to be invoked with interrupts >> enabled. The only reason we were disabling interrupts was to prevent >> recursive calls into the services on the same CPU, which will lead to >> deadlock. However, the only context where such invocations may occur >> legally is from efi-pstore via efivars, and that code has been updated >> to call a non-blocking alternative when invoked from a non-interruptible >> context. >> >> So instead, update the ordinary, blocking UEFI Runtime Services wrappers >> to execute with interrupts enabled. This aims to prevent excessive interrupt >> latencies on uniprocessor platforms with slow variable stores. > > Well, those excessive latencies would affect SMP platforms as well, just that > there are (usually) other CPUs free to do execution, right? >
Correct. > More fundamentally, this makes me nervous: > > > The UEFI spec allows Runtime Services to be invoked with interrupts > enabled. > > [...] > > So what really matters is not what the spec says, but how Windows executes > UEFI > firmware code in practice. > > If major versions of Windows calls UEFI firmware with interrupts disabled, > then > frankly I don't think we should interrupt them under Linux either, regardless > of > what the spec says ... > > Random firmware code getting interrupted by the OS changes timings and might > have > other side effects the firmware code might not expect - so the question is, > does > Windows already de facto allow the IRQ preemption of firmware calls? > Good question. I will try to find out. > Also, this: > >> - unsigned long flags; >> efi_status_t status; >> >> - spin_lock_irqsave(&efi_runtime_lock, flags); >> + BUG_ON(in_irq()); >> + >> + spin_lock(&efi_runtime_lock); > > ... how does crashing the kernel help debuggability? > > Please use WARN_ON_ONCE() - or in fact, this assert is probably not needed at > all, > as lockdep will warn about IRQ unsafe lock usage. > Actually, reading back the original thread, Matt had already identified this problem, and v2/v3 of this patch removed all of them but one, so thanks for spotting that. > I'd add comments to the efi_runtime_lock definition site explaining that this > is > never taken from IRQ contexts. > OK.
