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 17:42, <konrad.wilk@xxxxxxxxxx> wrote:
> On Wed, Nov 30, 2016 at 02:30:41AM -0700, Jan Beulich wrote:
>> >>> 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.
> 
> The page_alloc.c code walks through different order pages. Which means
> that if it can't find one within the requested order pages it will
> go one up (and so on). Eventually that means you do get the requested
> order pages, but they are not guaranteed to be order aligned (as they
> may be order aligned to a higher value).

But that's _better_ alignment than you asked for then.

Jan


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