Re: [Qemu-devel] [PATCH v7 0/5] IOMMU: intel_iommu support map and unmap notifications

[Alex Williamson]

On Wed, 30 Nov 2016 17:23:59 +0800
Peter Xu <address@hidden> wrote:

> On Mon, Nov 28, 2016 at 05:51:50PM +0200, Aviv B.D wrote:
> > * intel_iommu's replay op is not implemented yet (May come in different 
> > patch 
> >   set).
> >   The replay function is required for hotplug vfio device and to move 
> > devices 
> >   between existing domains.  
> 
> I am thinking about this replay thing recently and now I start to
> doubt whether the whole vt-d vIOMMU framework suites this...
> 
> Generally speaking, current work is throwing away the IOMMU "domain"
> layer here. We maintain the mapping only per device, and we don't care
> too much about which domain it belongs. This seems problematic.
> 
> A simplest wrong case for this is (let's assume cache-mode is
> enabled): if we have two assigned devices A and B, both belong to the
> same domain 1. Meanwhile, in domain 1 assume we have one mapping which
> is the first page (iova range 0-0xfff). Then, if guest wants to
> invalidate the page, it'll notify VT-d vIOMMU with an invalidation
> message. If we do this invalidation per-device, we'll need to UNMAP
> the region twice - once for A, once for B (if we have more devices, we
> will unmap more times), and we can never know we have done duplicated
> work since we don't keep domain info, so we don't know they are using
> the same address space. The first unmap will work, and then we'll
> possibly get some errors on the rest of dma unmap failures.
> 
> Looks like we just cannot live without knowing this domain layer.
> Because the address space is binded to the domain. If we want to sync
> the address space (here to setup a correct shadow page table), we need
> to do it per-domain.
> 
> What I can think of as a solution is that we introduce this "domain"
> layer - like a memory region per domain. When invalidation happens,
> it's per-domain, not per-device any more (actually I guess that's what
> current vt-d iommu driver in kernel is doing, we just ignored it - we
> fetch the devices that matches the domain ID). We can/need to maintain
> something different, like sid <-> domain mappings (we can do this as
> long as we are notified when context entries changed), per-domain
> mappings (just like per-device mappings that we are trying to build in
> this series, but what we really need is IMHO per domain one), etc.
> When device switches domain, we switch the IOMMU memory region
> accordingly.
> 
> Does this make any sense? Comments are greatly welcomed (especially
> from AlexW and DavidG).

It's been a bit since I've looked at VT-d emulation, but I certainly
remember that it's way more convoluted than I expected.  It seems like
a domain should create an AddressSpace and any devices assigned to that
domain should make use of that single address space, but IIRC VT-d
creates an address space per device, ie. per context entry.