linux-kernel - Re: [PATCH 5/8] x86/vm86: Add a separate config option for hardware IRQ handling

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Message-ID: <20150805085126.GB23169@gmail.com>
Date:	Wed, 5 Aug 2015 10:51:27 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 5/8] x86/vm86: Add a separate config option for hardware
 IRQ handling


* Brian Gerst <brgerst@...il.com> wrote:

> >> Disabling even less-used code that could have system stability impact. We've 
> >> discouraged user-mode drivers for a very long time.  Ironically, other than 
> >> being configured through the vm86 syscall, there isn't really anything 
> >> vm86-specific about it.  All it does is register an IRQ handler that sends a 
> >> signal to the task.
> >
> > So is this actually used by anything? Could we get away with disabling it, 
> > just to see whether anything cares?
> 
> My best guess would be some very old X11 drivers that needed interrupts to run 
> the Video BIOS code.

So let's keep it - but not complicate it with another layer of disabling logic. 
People that rely on legacies will enable vm86 as a single block - they won't 
necessarily know how deeply they rely on it.

What _would_ be useful is to have a 3-mode vm86 sysctl:

   1: enabled
   0: disabled
  -1: disabled permanently (one-shot disabling after bootup)

That way a distro can permanently disable vm86 for a particular bootup by setting 
it to -1 in /etc/sysctl.conf.

The kernel should default that setting to '0'.

Thanks,

	Ingo
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