Re: [Xen-devel] [PATCH RFC 1/2] xen/page_alloc: Add size_align parameter to provide MFNs which are size aligned.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.11.16 at 05:39, <konrad@xxxxxxxxxx> wrote:
> This is to support the requirement that exists in PV dom0
> when doing DMA requests:
> 
> "dma_alloc_coherent()
> [...]
> The CPU virtual address and the DMA address are both guaranteed to be
> aligned to the smallest PAGE_SIZE order which is greater than or equal
> to the requested size.  This invariant exists (for example) to guarantee
> that if you allocate a chunk which is smaller than or equal to 64
> kilobytes, the extent of the buffer you receive will not cross a 64K
> boundary."

So I'm having trouble understanding what it is that actually needs
fixing / changing here: Any order-N allocation will be order-N-aligned
already. Is your caller perhaps simply not passing in a large enough
order? And changing alloc_heap_pages(), which guarantees the
requested alignment already anyway (after all it takes an order
input, not a size one), looks completely pointless regardless of what
extra requirements you may want to put on the exchange hypercall.

Jan


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