linux-kernel - Re: swiotlb detection should be memory hotplug aware ?

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Date:	Fri, 12 Mar 2010 19:09:41 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	akataria@...are.com
CC:	Len Brown <lenb@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-acpi <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Petr Vandrovec <petr@...are.com>
Subject: Re: swiotlb detection should be memory hotplug aware ?

, Alok Kataria wrote:

Hi Alok,

> Hi,
>
> Looking at the current code swiotlb is initialized for 64bit kernels
> only when the max_pfn value is greater than 4G (MAX_DMA32_PFN value).
> So in cases when the initial memory is less than 4GB the kernel boots
> without enabling swiotlb, when we hotadd memory to such a kernel and go
> beyond the 4G limit, swiotlb is still disabled. As a result when any
> 32bit devices start using this newly added memory beyond 4G, the kernel
> starts spitting error messages like below or in some cases it causes
> kernel panics.

Yes seems like a real problem.

>
> 1. Enable swiotlb for all 64bit kernels which have memory hot-add
> support.

I don't think that's a good idea. It would enable it everywhere on
distributions which compile with hotadd. Need (2)

> 2. Instead of checking the max_pfn value in pci_swiotlb_detect, check
> for max_hotpluggable_pfn (or some such) value. Though I don't see such a
> value readily available. I could parse the SRAT and get hotplug memory
> information but that will make swiotlb detection logic a little too
> complex. A quick look around srat_xx.c files and the acpi_memhotplug
> module didn't find any useful API that could be used directly either.
> So was wondering if any of you are aware of an easy way to get such
> information ?

I have a patchkit to revamp the SRAT parsing to store the hotadd information
more efficiently (the current way is pretty dumb)  I need to repost that.

With that it would be relatively easy to do I think.

-Andi
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