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SubjectRe: [LKML] Re: swiotlb detection should be memory hotplug aware ?
On Fri, Mar 12, 2010 at 07:09:41PM -0800, Andi Kleen wrote:
> , 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

There is a late mechanism to do kickoff the SWIOTLB. Perhaps the hot-add
could use swiotlb_init_late and start up the SWIOTLB?

> 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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