linux-kernel - Re: [PATCH v6 00/18] ARM: Add support for the Large Physical Address Extensions

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Date:	Wed, 25 May 2011 13:43:11 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Nicolas Pitre <nico@...xnic.net>,
	David Brown <davidb@...eaurora.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 00/18] ARM: Add support for the Large Physical Address Extensions

On 25 May 2011 12:22, Catalin Marinas <catalin.marinas@....com> wrote:
> On Wed, 2011-05-25 at 12:10 +0100, Arnd Bergmann wrote:
>> On Wednesday 25 May 2011, Nicolas Pitre wrote:
>> > > Do you expect non LPAE targets to be able to boot with these changes
>> > > applied (and LPAE enabled)?  I am able to build this tree for the
>> > > MSM8660 (with a minor patch below), but it fails to boot with LPAE
>> > > enabled.  It seems to work fine with LPAE not enabled.
>> >
>> > I wouldn't expect a LPAE kernel to boot on non LPAE capable hardware.
>> >
>> > And I don't think we'll ever try to support both LPAE and non-LPAE modes
>> > in the same kernel binary either since this has implications all over
>> > the place in core kernel mm code.
>>
>> Just for information and to complete the picture, what other of these
>> combinations are possible?
>>
>> * Non-LPAE kernel on LPAE-capable hardware
>
> Works fine. The LPAE-capable processors like A15 support the classic
> page table format as well, it's only a configuration bit to switch to
> LPAE.
>
>> * LPAE kernel as KVM guest of LPAE kernel
>
> Should work when KVM for A15 becomes available
>
>> * non-LPAE kernel as KVM guest of LPAE kernel
>
> Same as above. KVM shouldn't care much about the underlying OS, it just
> sets up the IPA (Intermediate Physical Address) translations and you can
> have classic page tables on top.

Just a clarification here - KVM with support for LPAE also requires
the virtualisation extensions (which are present in Cortex-A15). The
virtualisation extensions require LPAE in hardware but I'm not sure
the opposite is true.

-- 
Catalin
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