Re: [PATCH 2/6] system/physmem: IOMMU: Invoke the translate_size function if it is implemented

[Peter Xu]

On Fri, Oct 27, 2023 at 11:28:36AM +0800, Ethan Chen wrote:
> On Thu, Oct 26, 2023 at 10:20:41AM -0400, Peter Xu wrote:
> > Could you elaborate why is that important?  In what use case?
> I was not involved in the formulation of the IOPMP specification, but I'll try
> to explain my perspective. IOPMP use the same the idea as PMP. "The matching 
> PMP entry must match all bytes of an access, or the access fails."
> 
> > 
> > Consider IOVA mapped for address range iova=[0, 4K] only, here we have a
> > DMA request with range=[0, 8K].  Now my understanding is what you want to
> > achieve is don't trigger the DMA to [0, 4K] and fail the whole [0, 8K]
> > request.
> > 
> > Can we just fail at the latter DMA [4K, 8K] when it happens?  After all,
> > IIUC a device can split the 0-8K DMA into two smaller DMAs, then the 1st
> > chunk can succeed then if it falls in 0-4K.  Some further explanation of
> > the failure use case could be helpful.
> 
> IOPMP can only detect partially hit in an access. DMA device will split a 
> large DMA transfer to small DMA transfers base on target and DMA transfer 
> width, so partially hit error only happens when an access cross the boundary.
> But to ensure that an access is only within one entry is still important. 
> For example, an entry may mean permission of a device memory region. We do 
> not want to see one DMA transfer can access mutilple devices, although DMA 
> have permissions from multiple entries.

I was expecting a DMA request can be fulfilled successfully as long as the
DMA translations are valid for the whole range of the request, even if the
requested range may include two separate translated targets or more, each
point to different places (either RAM, or other devicie's MMIO regions).

AFAIK currently QEMU memory model will automatically split that large
request into two or more smaller requests, and fulfill them separately by
two/more IOMMU translations, with its memory access dispatched to the
specific memory regions.

The example you provided doesn't seem to be RISCV specific.  Do you mean it
is a generic requirement from PCI/PCIe POV, or is it only a restriction of
IOPMP?  If it's a valid PCI restriction, does it mean that all the rest
IOMMU implementations in QEMU currently are broken?

[copy Michael and Igor]

Thanks,

-- 
Peter Xu