Re: [Xen-devel] [PATCH-REWORKED] Added xen_is_contiguous_region

["Jambunathan K" <kjambunathan@xxxxxxxxx>]

>  You can check whether a memory region straddles a page boundary yourself
> without needing an external helper function. Also you should have a pretty
> good idea whether a piece of memory you allocated will pass the
> contiguous_bitmap test:

Agreed.

But I had seen a crash in DomU (with no swiotlb) when we try to pci_map_single
a alloc_skb buffer.

In Xen-3.0.4, alloc_skb in all probability allocs a contig buffer but
it wouldn't
guarantee me the same. With Xen-3.0.5, with bit-width based allocator
we are much
more likely to succeed compared to system-wide functions that cater to least
common denominator.

> But anyway — I thought the issue was your DMA limit of 32GB? Why should
> page-straddling buffers be a problem at all: your hardware probably supports
> scatter-gather DMA, right?

We don't do scatter-gather DMA as yet.

Jambunathan K.

On 4/14/07, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
>  On 13/4/07 20:56, "Jambunathan K" <kjambunathan@xxxxxxxxx> wrote:
>
>
> Using the new xen_is_contiguous_region() would enable our driver to fall back 
> upon it's own pool.
>
>  This is my first patch to Xen. If you approve of the patch, I will resubmit 
> the changes as required by you.
>
>  You can check whether a memory region straddles a page boundary yourself 
> without needing an external helper function. Also you should have a pretty good 
> idea whether a piece of memory you allocated will pass the contiguous_bitmap 
> test: if you called pci_alloc_consistent(), or xen_create_contiguous_region(), 
> then it will; if you simply kmalloc()ed or get_free_pages()ed a region then it 
> will not. If you are getting buffers passed to you from elsewhere in the 
> kernel, I guess it depends what kind of device you're talking about, but it's 
> pretty likely to be the case that if you are passed page-straddling buffers 
> that you are going to need to use a bounce buffer.
>
>  But anyway — I thought the issue was your DMA limit of 32GB? Why should 
> page-straddling buffers be a problem at all: your hardware probably supports 
> scatter-gather DMA, right?
>
>   -- Keir

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