Re: [PATCH v1 0/4] vfio/iommu: Flag to allow userspace to set DMA buffers system cacheable
From: Robin Murphy
Date: Tue Aug 27 2024 - 13:31:41 EST
On 27/08/2024 12:17 am, Jason Gunthorpe wrote:
On Mon, Aug 26, 2024 at 11:04:47AM -0600, Alex Williamson wrote:
On Mon, 26 Aug 2024 07:16:37 +0000
Manoj Vishwanathan <manojvishy@xxxxxxxxxx> wrote:
Hi maintainers,
This RFC patch introduces the ability for userspace to control whether
device (DMA) buffers are marked as cacheable, enabling them to utilize
the system-level cache.
The specific changes made in this patch are:
* Introduce a new flag in `include/linux/iommu.h`:
* `IOMMU_SYS_CACHE` - Indicates if the associated page should be cached in the system's cache hierarchy.
* Add `VFIO_DMA_MAP_FLAG_SYS_CACHE` to `include/uapi/linux/vfio.h`:
You'll need a much better description of what this is supposed to do
when you resend it.
IOMMU_CACHE already largely means that pages should be cached.
So I don't know what ARM's "INC_OCACHE" actually is doing. Causing
writes to land in a cache somewhere in hierarchy? Something platform
specific?
Kinda both - the Inner Non-Cacheable attribute means it's still
fundamentally non-snooping and non-coherent with CPU caches, but the
Outer Write-back Write-allocate attribute can still control allocation
in a system cache downstream of the point of coherency if the platform
is built with such a thing (it's not overly common).
However, as you point out, this is in direct conflict with the Inner
Write-back Write-allocate attribute implied by the IOMMU_CACHE which
VFIO adds in further down in vfio_iommu_map(). Plus the way it's
actually implemented in patch #2, IOMMU_CACHE still takes precedence and
would lead to this new value being completely ignored, so there's a lot
which smells suspicious here...
Thanks,
Robin.
I have no idea. By your description it sounds similar to the
x86 data placement stuff, whatever that was called, and the more
modern TPH approach.
Jason