Date: Mon, 18 May 2015 17:41:08 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Salter <msalter@redhat.com>
Cc: Matt Fleming <matt.fleming@intel.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
        Hanjun Guo <hanjun.guo@linaro.org>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: support ACPI tables outside of kernel RAM
Message-ID: <20150518164108.GH21251@e104818-lin.cambridge.arm.com>
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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?

-- 
Catalin
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