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SubjectRe: swiotlb detection should be memory hotplug aware ?
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Hi Andi, 

On Fri, 2010-03-12 at 19:09 -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
> more efficiently (the current way is pretty dumb) I need to repost that.

Can you please send me any pointers to that patch series. Also am just
curious to know about any merge plans for that patchkit.

Thanks,
Alok
>
> With that it would be relatively easy to do I think.
>
> -Andi



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