Re: [Qemu-devel] [PATCH] vfio: Align iova also to IOMMU page size

[Peter Maydell <peter.maydell@linaro.org>]

On 18 November 2015 at 22:04, Alex Williamson
<alex.williamson@redhat.com> wrote:
> On Tue, 2015-11-17 at 10:46 +0300, Pavel Fedin wrote:
>> On some architectures TARGET_PAGE_ALIGN() is not enough to get the right
>> alignment. For example on ARM TARGET_PAGE_BITS is 10 because some old CPUs
>> support 1K page size, while minimum SMMU page size is 4K.

> I don't understand how this is supposed to work, if we align to a larger
> size than the processor, then there are processor size pages of RAM than
> could be handed out as DMA targets for devices, but we can't map them
> through the IOMMU.  Thus if the guest tries to use them, we get IOMMU
> faults in the host and likely memory corruption in the guest because the
> device can't read or write to the page it's supposed to.  This doesn't
> seem like the right solution.  Thanks,

There are a number of different interesting page sizes here:
 * the host kernel page size
 * the target CPU architecture's worst-case smallest page size
 * the page size the guest kernel is actually using at the moment
   (consider a 4K-page guest kernel on a 64K-page host kernel)

These don't necessarily have to all be the same. I would
expect VFIO to be interested in the host kernel page size,
not TARGET_PAGE_ALIGN. It might also be interested in the
in-practice guest kernel page settings, but you can't actually
determine those from outside. (In general non-TCG code should
probably not try to use the TARGET_PAGE_* constants.)

thanks
-- PMM