linux-kernel - Re: [GIT PULL] EFI changes for v3.18

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Message-ID: <20140929154142.GO5430@worktop>
Date:	Mon, 29 Sep 2014 17:41:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [GIT PULL] EFI changes for v3.18

On Mon, Sep 29, 2014 at 04:00:09PM +0100, Matt Fleming wrote:
> On Mon, 29 Sep, at 04:05:16PM, Peter Zijlstra wrote:
> > 
> > OMFG what a trainwreck... if they are reentrant like that, a lock isn't
> > going to help you in any way. The implementation of these calls must be
> > lockfree otherwise they cannot possibly be correct.
>  
> The only way these services are going to be called is if we (the OS)
> invoke them. These NMI shenanigans we're playing in the locking
> functions are actually for our benefit, not the firmware's.
> 
> > Conditional locking like above is just plain broken, disgusting doesn't
> > even begin to cover it. Full NAK on this. If this is required by the EFI
> > spec someone needs to pull their head from their arse and smell the real
> > world.
> 
> The conditional locking isn't intended to conform to the spec, it's
> intended to write a pstore object to the EFI variable NVRAM with our
> last dying breath, even if someone was in the middle of a SetVariable()
> call. All the spec says is that, if we're in a non-recoverable state,
> it's OK to do that. Whether that'll be implemented correctly across a
> range of firmware implementations is another matter.
> 
> In fact, maybe we shouldn't support that lockless access at all. I've no
> huge problem changing the code in efi_pstore_write() to bail if we can't
> grab the lock, so that we can be serialized all of the time.
> 
> That would certainly make the runtime lock code much simpler.

Right, like we talked about on IRC, we need to either drop all from NMI
stuff or do the trylock-from-NMI thing you suggested and have a runtime
test to make sure that all actually works.

Because just not having any serialization is relying on the firmware to
not screw itself, which I think is unsound, esp. given that Windows is
unlikely to rely on this and we all know the quality of implementation,
esp. outside of what Windows does.
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