Re: [PATCH v2 2/2] vring: Force use of DMA API for ARM-based systems

[Will Deacon <will.deacon@arm.com>]

On Fri, Jan 13, 2017 at 08:23:35PM +0200, Michael S. Tsirkin wrote:
> On Fri, Jan 13, 2017 at 05:21:54PM +0000, Will Deacon wrote:
> > On Fri, Jan 13, 2017 at 06:46:32PM +0200, Michael S. Tsirkin wrote:
> > > On Fri, Jan 13, 2017 at 09:25:22AM +0000, Will Deacon wrote:
> > > > On Fri, Jan 13, 2017 at 12:12:56AM +0200, Michael S. Tsirkin wrote:
> > > > > I'd rather people didn't use SMMU with legacy devices.
> > > > 
> > > > I'm afraid we've been doing that for two years and the model already
> > > > exists in a mature state, being actively used for development and
> > > > validation by ARM and our partners. One of the big things its used for
> > > > is to develop SMMU and GIC (our interrupt controller) code with PCI, so
> > > > dropping the SMMU from the picture isn't an option.
> > > 
> > > Oh so this fixes a regression?  This is something I didn't realize.
> > 
> > Yes, thanks. The regression came about because we implemented SMMU-backed
> > DMA ops and only then was it apparent that the virtio stuff was bypassing
> > even with translation enabled (because it wasn't using the DMA API).
> 
> Could you point out a commit ID?

There has been a fair amount of work in this area recently, but you're
probably after something like 876945dbf649 ("arm64: Hook up IOMMU dma_ops")
as the culprit, which is the point at which we started to swizzle DMA
ops for devices upstream of an SMMU automatically.

> > > A "Fixes:" tag can't hurt here.  I then wonder
> > > might DMA ops ever use a DMA address which isn't a physical address
> > > from QEMU point of view? If that happens, this hack breaks
> > > because in legacy mode QEMU still uses the GPA.
> > 
> > If QEMU doesn't advertise an SMMU, then it will work fine with the GPA,
> > because we won't swizzle the DMA ops for the master device. If QEMU does
> > advertise an SMMU, then we'll allocate DMA addresses to fit within the
> > the intersection of the SMMU aperture and device's DMA mask.
> 
> 
> Right but doesn't just poking from qemu into phys addresses work
> anymore? It used to ...

Provided that there's no SMMU, then it will continue to work. and my
understanding (from talking to Peter Maydell) is that qemu doesn't model
an SMMU for ARM-based machines.

Will