linux-kernel - Re: [PATCH] x86-64: use EFI to deal with platform wall clock

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Message-ID: <20120516130700.GA21499@srcf.ucam.org>
Date:	Wed, 16 May 2012 14:07:00 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	mingo@...e.hu, tglx@...utronix.de, matt.fleming@...ux.intel.com,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [PATCH] x86-64: use EFI to deal with platform wall clock

On Wed, May 16, 2012 at 01:59:57PM +0100, Jan Beulich wrote:
> >>> On 16.05.12 at 14:39, Matthew Garrett <mjg59@...f.ucam.org> wrote:
> > Could you elaborate on that a little?
> 
> There are systems where RAM on individual nodes is always
> starting at e.g. a 1Tb boundary. Obviously there can (at
> least theoretically) be anything in between, and hence
> assuming that __va() is usable here is simply wrong, as likely
> no mapping was created at all for the hole space (or if there
> is one, it would conflict with the one to be established here
> in the EfiMemoryMappedIO case).

Ok, that does sound like it needs fixing.

> > Platforms don't correctly deal with the case where you make physical 
> > calls after ExitBootServices(). We tried running in physical mode. It 
> > simply doesn't work.
> 
> Interesting, especially as we're using physical mode exclusively so
> far in Xen. That's a platform issue though, and working around
> it by (silently!) sacrificing other functionality is questionable imo.
> It should at best be an option (default off), so that on systems
> where physical mode works, kexec can work too.

There was a patchset posted that provided that option. We experimented 
with it in RHEL for a while and found that physical mode simply isn't 
reliable - no other OS uses it, so it's entirely untested on most 
platforms.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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