Re: [Qemu-devel] [PATCH for-2.9 2/2] intel_iommu: extend supported guest aw to 48 bits

[Peter Xu <peterx@redhat.com>]

On Mon, Dec 12, 2016 at 12:35:44PM -0700, Alex Williamson wrote:
> On Mon, 12 Dec 2016 10:01:15 +0800
> Peter Xu <peterx@redhat.com> wrote:
> 
> > On Sun, Dec 11, 2016 at 05:13:45AM +0200, Michael S. Tsirkin wrote:
> > > On Wed, Dec 07, 2016 at 01:52:45PM +0800, Peter Xu wrote:  
> > > > Previously vt-d codes only supports 39 bits iova address width. It won't
> > > > be hard to extend it to 48 bits.
> > > > 
> > > > After enabling this, we should be able to map larger iova addresses.
> > > > 
> > > > To check whether 48 bits aw is enabled, we can grep in the guest dmesg
> > > > with line: "dmar: Host address width 48" (previously it was 39).
> > > > 
> > > > Signed-off-by: Peter Xu <peterx@redhat.com>  
> > > 
> > > I suspect we can't do this for old machine types.
> > > Need to behave in compatible ways.  
> > 
> > Sure. I can do that.
> > 
> > Btw, is vt-d iommu still in experimental stage? I am just thinking
> > whether it'll be overkill we add lots of tunables before we have one
> > stable and mature vt-d emulation.
> > 
> > > Also, is 48 always enough? 5 level with 57 bits
> > > is just around the corner.  
> > 
> > Please refer to the discussion with Jason - looks like vt-d spec
> > currently supports only 39/48 bits address width? Please correct if I
> > made a mistake.
> > 
> > > And is it always supported? for things like vfio
> > > to work, don't we need to check what does host support?  
> > 
> > Hmm, yes, we should do that. But until now, we still don't have a
> > complete vfio support. IMHO we can postpone this issue until vfio is
> > fully supported.
> 
> I'm not sure how the vIOMMU supporting 39 bits or 48 bits is directly
> relevant to vfio, we're not sharing page tables.  There is already a
> case today, without vIOMMU that you can make a guest which has more
> guest physical address space than the hardware IOMMU by overcommitting
> system memory.  Generally this quickly resolves itself when we start
> pinning pages since the physical address width of the IOMMU is
> typically the same as the physical address width of the host system
> (ie. we exhaust the host memory).

Hi, Alex,

Here does "hardware IOMMU" means the IOMMU iova address space width?
For example, if guest has 48 bits physical address width (without
vIOMMU), but host hardware IOMMU only supports 39 bits for its iova
address space, could device assigment work in this case?

Thanks,

-- peterx