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Re: [Xen-devel] [PATCH RFC 0/3] introduce XENMEM_get_dma_buf and XENMEM_put_dma_buf

On Thu, 2013-08-01 at 10:41 +0100, David Vrabel wrote:
> On 31/07/13 18:57, Stefano Stabellini wrote:
> > Hi all,
> > this patch series introduces two new hypercalls to allow autotranslate
> > guests to allocate a contiguous buffer in machine addresses.
> > The XENMEM_get_dma_buf returns the mfns and makes sure to pin the pages
> > so that the hypervisor won't change their p2m mappings while in use.
> > XENMEM_put_dma_buf simply unpins the pages.
> Can you expand on what circumstances the hypervisor would otherwise
> adjust the p2m?  How has x86 avoided these problems?

Page sharing, CoW, Dario's forthcoming THP like stuff.

x86 avoids this by not supporting any of that for PV guests and by using
an IOMMU for HVM guests.

On ARM IOMMUs are currently rare enough that we need a software
fallback, which is what this series is about.

We actually only need the MFN lookup behaviour currently but the pinning
behaviour is somewhat forward looking.

I don't know if PVH on x86 is going to trip over similar issues but I
expect that IOMMUs are prevalent enough that they can be a prerequisite.


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