On 15.01.25 13:49, Jason Gunthorpe wrote:
On Wed, Jan 15, 2025 at 02:39:55PM +1100, Alexey Kardashevskiy wrote:
The thread was talking about the built-in support in iommufd to split
mappings.
Just to clarify - I am talking about splitting only "iommufd areas", not
large pages.
In generality it is the same thing as you cannot generally guarantee
that an area split doesn't also cross a large page.
If all IOMMU PTEs are 4k and areas are bigger than 4K => the hw
support is not needed to allow splitting. The comments above and below seem
to confuse large pages with large areas (well, I am consufed, at least).
Yes, in that special case yes.
That built-in support is only accessible through legacy APIs
and should never be used in new qemu code. To use that built in
support in new code we need to build new APIs.
Why would not IOMMU_IOAS_MAP/UNMAP uAPI work? Thanks,
I don't want to overload those APIs, I prefer to see a new API that is
just about splitting areas. Splitting is a special operation that can
fail depending on driver support.
So we'd just always perform a split-before-unmap. If split fails, we're
in trouble, just like we would be when unmap would fail.
If the split succeeded, the unmap will succeed *and* be atomic.
That sounds reasonable to me and virtio-mem could benefit from that as well.
--
Cheers,
David / dhildenb
