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

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Message-ID: <20160203120137.GE2597@codeblueprint.co.uk>
Date:	Wed, 3 Feb 2016 12:01:37 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Ingo Molnar <mingo@...nel.org>, "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>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Subject: Re: [PATCH 07/14] efi: runtime-wrappers: Run UEFI Runtime Services
 with interrupts enabled

On Wed, 03 Feb, at 12:33:10PM, Ard Biesheuvel wrote:
> 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.
 
It was indeed. Additionally I didn't want the EFI material to miss the
merge window again.

> > 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?)

Sure, I can help out with that. Hit me up on IRC. I'm also looping in
Sai who has done OVMF hacking for OS diagnostics in the past.