linux-kernel - Re: [PATCH] arm64: support ACPI tables outside of kernel RAM

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Message-ID: <CAKv+Gu8wXrCnnDEw2gpJ9e1qixkVvBCsUf0wLorsdLJwknZELA@mail.gmail.com>
Date:	Mon, 18 May 2015 18:49:28 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Mark Salter <msalter@...hat.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: support ACPI tables outside of kernel RAM

On 18 May 2015 at 18:41, Catalin Marinas <catalin.marinas@....com> wrote:
> On Mon, May 18, 2015 at 09:58:45AM -0400, Mark Salter wrote:
>> On Mon, 2015-05-18 at 12:11 +0100, Catalin Marinas wrote:
>> > On Thu, May 14, 2015 at 10:22:53AM -0400, Mark Salter wrote:
>> > > There is no guarantee that ACPI tables will be located in RAM linearly
>> > > mapped by the kernel. This could be because UEFI placed them below the
>> > > kernel image or because mem= places them beyond the reach of the linear
>> > > kernel mapping. Even though these tables are outside the linear mapped
>> > > RAM, they still need to be accessed as normal memory in order to support
>> > > unaligned accesses from ACPI code. In this case, the page_is_ram() test
>> > > in acpi_os_ioremap() is not sufficient.
>> >
>> > And can we not simply add the rest of the RAM to the resource list as
>> > "System RAM" without being part of memblock?
>>
>> If it is in "System RAM", then it needs a valid pfn and struct page.
>> Parts of the kernel expect that (page_is_ram(), memory hotplug, etc).
>
> OK, I had the impression that we could get away with this.
>
>> > > Additionally, if the table spans multiple pages, it may fall partially
>> > > within the linear map and partially without. If the table overlaps the
>> > > end of the linear map, the test for whether or not to use the existing
>> > > mapping in ioremap_cache() could lead to a panic when ACPI code tries
>> > > to access the part beyond the end of the linear map. This patch
>> > > attempts to address these problems.
>> >
>> > That's a problem with ioremap_cache() that should be fixed independently.
>>
>> I can submit that separately if you prefer.
>
> Yes, please.
>
>> > Ideally, I'd like to see the ACPI code use different APIs to distinguish
>> > between table access in RAM and device access, so that we don't have to
>> > guess whether the page is RAM or not.
>>
>> I don't think the ACPI code has enough info to make that decision, but
>> I'm not sure honestly.
>
> Do we have a guarantee that UEFI tells the kernel about the whole RAM?
>

Yes, the UEFI memory map must describe all of RAM, no matter how it is
used. I may also describe some MMIO regions, but typically only
regions that it needs itself to implement the UEFI Runtime Services
(e.g., RTC base address, NOR flash for the variable store)

So we could potentially query the UEFI memory map directly to find out
whether some otherwise unqualified region is backed by RAM or not,
although I'd prefer some intermediate data structure (such as the
physmem memblock table) if we go that route.

-- 
Ard.
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