Re: [PATCH] efi: arm/arm64: allow SetVirtualAddressMap() to be omitted

From: Lee Jones <lee.jones@linaro.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: mark.rutland@arm.com, linux-efi@vger.kernel.org,
	Heinrich Schuchardt <xypron.glpk@gmx.de>,
	will.deacon@arm.com, Alexander Graf <agraf@suse.de>,
	Leif Lindholm <leif.lindholm@linaro.org>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	james.morse@arm.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] efi: arm/arm64: allow SetVirtualAddressMap() to be omitted
Date: Fri, 1 Feb 2019 08:06:52 +0000	[thread overview]
Message-ID: <20190201080652.GA783@dell> (raw)
In-Reply-To: <20190126102207.29488-1-ard.biesheuvel@linaro.org>

On Sat, 26 Jan 2019, Ard Biesheuvel wrote:

> The UEFI spec revision 2.7 errata A section 8.4 has the following to
> say about the virtual memory runtime services:
> 
>   "This section contains function definitions for the virtual memory
>   support that may be optionally used by an operating system at runtime.
>   If an operating system chooses to make EFI runtime service calls in a
>   virtual addressing mode instead of the flat physical mode, then the
>   operating system must use the services in this section to switch the
>   EFI runtime services from flat physical addressing to virtual
>   addressing."
> 
> So it is pretty clear that calling SetVirtualAddressMap() is entirely
> optional, and so there is no point in doing so unless it achieves
> anything useful for us.
> 
> This is not the case for 64-bit ARM. The native mapping used by the OS
> is arbitrarily converted into another permutation of userland addresses
> (i.e., bits [63:48] cleared), and the runtime code could easily deal
> with the original layout in exactly the same way as it deals with the
> converted layout. However, due to constraints related to page size
> differences if the OS is not running with 4k pages, and related to
> systems that may expose the individual sections of PE/COFF runtime
> modules as different memory regions, creating the virtual layout is a
> bit fiddly, and requires us to sort the memory map and reason about
> adjacent regions with identical memory types etc etc.
> 
> So the obvious fix is to stop calling SetVirtualAddressMap() altogether
> on arm64 systems. However, to avoid surprises, which are notoriously
> hard to diagnose when it comes to OS<->firmware interactions, let's
> start by making it an opt-out feature, and implement support for the
> 'efi=novamap' kernel command line parameter on ARM and arm64 systems.
> 
> (Note that 32-bit ARM generally does require SetVirtualAddressMap() to be
> used, given that the physical memory map and the kernel virtual address
> map are not guaranteed to be non-overlapping like on arm64. However,
> having support for efi=novamap,noruntime on 32-bit ARM, combined with
> the recently proposed support for earlycon=efi, is likely to be useful
> to diagnose boot issues on such systems if they have no accessible serial
> port)
> 
> Cc: Alexander Graf <agraf@suse.de>
> Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
> Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
> Cc: Leif Lindholm <leif.lindholm@linaro.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Tested-by: Lee Jones <lee.jones@linaro.org>

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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