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Date: Tue, 17 Nov 2015 06:33:46 +0100
Message-ID: <CAKv+Gu8H5oqrke-TpMj9i00=OJpi82NKySF40yVQoOhXbaQvXA@mail.gmail.com>
Subject: Re: [PATCH v2 11/12] ARM: wire up UEFI init and runtime support
From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
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To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, "linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>, Will Deacon <will.deacon@arm.com>, Grant Likely <grant.likely@linaro.org>, Catalin Marinas <catalin.marinas@arm.com>, Mark Rutland <mark.rutland@arm.com>, Leif Lindholm <leif.lindholm@linaro.org>, Roy Franz <roy.franz@linaro.org>, Mark Salter <msalter@redhat.com>, Ryan Harkin <ryan.harkin@linaro.org>, Andrew Morton <akpm@linux-foundation.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>

On 16 November 2015 at 20:48, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Mon, Nov 16, 2015 at 08:04:00PM +0100, Ard Biesheuvel wrote:
>> OK. So you mean set TTBR to the zero page, perform the TLB flush and
>> only then switch to the new page tables?
>
> Not quite.
>
> If you have global mappings below TASK_SIZE, you would need this
> sequence when switching either to or from the UEFI page tables:
>
> - switch to another set of page tables which only map kernel space
>   with nothing at all in userspace.
> - flush the TLB.
> - switch to your target page tables.
>

Doh. I am so used to always having two TTBR's available, but indeed,
we shouldn't pull the rug from under our feet.

> As I say in response to one of your other patches, it's probably
> much easier to avoid any global mappings below TASK_SIZE.

Let me look into that.

Thanks,
Ard.

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