Re: [PATCH] xen-swiotlb: exchange memory with Xen only when pages are contiguous

[Christoph Helwig]

On Thu, Oct 25, 2018 at 11:56:02AM -0700, Joe Jin wrote:
> I just discussed this patch with Boris in private, his opinions(Boris,
> please correct me if any misunderstood) are:
> 
> 1. With/without the check, both are incorrect, he thought we need to
>    prevented unalloc'd free at here. 
> 2. On freeing, if upper layer already checked the memory was DMA-able,
>    the checking at here does not make sense, we can remove all checks.
> 3. xen_create_contiguous_region() and xen_destroy_contiguous_region()
>    to come in pairs.
> 
> For #1 and #3, I think we need something associate it, like a list, on
> allocating, add addr to it, on freeing, check if in the list.

Is there any way to figure out based on an address if the exchange
operation happened?

> For #2, I'm was not found anywhere validated the address on 
> dma_free_coherent() callpath, not just xen-swiotlb.

At least for simple direct mappings there is no easy way to verify that
without keeping a list, and for some of the ops that do vmap like
operations we have basic santiy checks, but nothing that really catches
a wrong free.