From: Dongli Zhang <redacted> Sent: Friday, March 4, 2022 10:28 AM

Hi Michael,

On 3/4/22 10:12 AM, Michael Kelley (LINUX) wrote:

quoted

From: Christoph Hellwig <hch@lst.de> Sent: Tuesday, March 1, 2022 2:53 AM

quoted

Power SVM wants to allocate a swiotlb buffer that is not restricted to low memory for
the trusted hypervisor scheme.  Consolidate the support for this into the swiotlb_init
interface by adding a new flag.

Hyper-V Isolated VMs want to do the same thing of not restricting the swiotlb
buffer to low memory.  That's what Tianyu Lan's patch set[1] is proposing.
Hyper-V synthetic devices have no DMA addressing limitations, and the
likelihood of using a PCI pass-thru device with addressing limitations in an
Isolated VM seems vanishingly small.

So could use of the SWIOTLB_ANY flag be generalized?  Let Hyper-V init
code set the flag before swiotlb_init() is called.  Or provide a CONFIG
variable that Hyper-V Isolated VMs could set.

I used to send 64-bit swiotlb, while at that time people thought it was the same
as Restricted DMA patchset.

https://lore.kernel.org/all/20210203233709.19819-1-dongli.zhang@oracle.com/ (local)

However, I do not think Restricted DMA patchset is going to supports 64-bit (or
high memory) DMA. Is this what you are looking for?

Yes, it looks like your patchset would do what we want for Hyper-V Isolated
VMs, but it is a more complex solution than is needed.  My assertion is that
in some environments, such as Hyper-V Isolated VMs, we're willing to assume
all devices are 64-bit DMA capable, and to stop carrying the legacy baggage.
Bounce buffering is used for a different scenario (memory encryption), and
the bounce buffers can be allocated in high memory.   There's no need for a
2nd swiotlb buffer.

Michael