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

[Ethan Chen]

On Mon, Oct 30, 2023 at 11:02:30AM -0400, Peter Xu wrote:
> On Mon, Oct 30, 2023 at 02:00:54PM +0800, Ethan Chen wrote:
> > On Fri, Oct 27, 2023 at 12:13:50PM -0400, Peter Xu wrote:
> > > Add cc list.
> > > 
> > > 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).
> > 
> > IOPMP is used to check DMA translation is vaild or not. In IOPMP 
> > specification
> > , a translation access more than one entry is not invalid.
> > Though it is not recommand, user can create an IOPMP entry contains mutiple
> > places to make this kind translations valid.
> > 
> > > > 
> > > > 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.
> > 
> > Because of requests may be split, I need a method to take the original 
> > request
> > information to IOPMP.
> 
> I'm not sure whether translate() is the "original request" either.  The
> problem is QEMU can split the request for various reasons already, afaict.
> 
> For example, address_space_translate_internal() has this:
> 
>     if (memory_region_is_ram(mr)) {
>         diff = int128_sub(section->size, int128_make64(addr));
>         *plen = int128_get64(int128_min(diff, int128_make64(*plen)));
>     }
> 
> Which can already shrink the request size from the caller before reaching
> translate().  So the length passed into translate() can already be
> modified.
> 
> Another thing is, we have two other common call sites for translate():
> 
>         memory_region_iommu_replay
>         address_space_translate_for_iotlb
> 
> I'm not sure whether you've looked into them and think they don't need to
> be trapped: at least memory_region_iommu_replay() looks all fine in this
> regard because it always translate in min page size granule.  But I think
> the restriction should apply to all translate()s.
> 
> translate_size() is weird on its own. If the only purpose is to pass the
> length into translate(), another option is to add that parameter into
> current translate(), allowing the implementation to ignore it.  I think
> that'll be better, but even if so, I'm not 100% sure it'll always do what
> you wanted as discussed above.

It seems that there are too many things that have not been considered in my 
current method. I am doing the revision that no new translation function but 
adding start address and end address to MemTxAttrs. 

Since attrs_to_index() only return one interger, IOPMP attrs_to_index() will 
copy the address range to its device state and then handle the translate(). 

Thanks,
Ethan Chen