linux-kernel - Re: [PATCH 07/14] efi: runtime-wrappers: Run UEFI Runtime Services with interrupts enabled

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Date:	Wed, 3 Feb 2016 12:33:10 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Matt Fleming <matt@...eblueprint.co.uk>,
	"H . Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 07/14] efi: runtime-wrappers: Run UEFI Runtime Services
 with interrupts enabled

On 3 February 2016 at 11:58, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>
>> > 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.
>
> Note that if there's a reasonable (but not 100%) case in favor of keeping irqs
> enabled, we can try your patch, with the possibility that we might have to revert
> it, should it cause problems.
>

I think this might have been the reason Matt wanted this in -next
early, but I will let him confirm whether that was the case.

> In practice we probably already interrupt EFI services with NMI interrupts, which
> can be pretty heavy as well if they for example generate printks.
>
> So I'm not against this change in a strong fashion - I'm just a bit cautious and
> it would be nice to know how Windows behaves here.
>

I am not sure how yet, but I am going to try and figure out what
Windows does. I suppose hacking OVMF to record some IRQ mask
information when RT services are being invoked should be sufficient,
but I am going to need some help from someone that understands OVMF
and x86 (Matt?)