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

[Peter Xu <peterx@redhat.com>]

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On Fri, Oct 27, 2023 at 12:02:24PM -0400, Peter Xu wrote:
> 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

-- 
Peter Xu