linux-kernel - Re: [PATCH v5 11/13] xen: introduce xen_alloc/free_coherent

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Message-ID: <alpine.DEB.2.02.1309061706480.6397@kaball.uk.xensource.com>
Date:	Fri, 6 Sep 2013 17:09:52 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Catalin Marinas <catalin.marinas@....com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"Ian.Campbell@...rix.com" <Ian.Campbell@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Subject: Re: [PATCH v5 11/13] xen: introduce xen_alloc/free_coherent_pages

On Fri, 6 Sep 2013, Catalin Marinas wrote:
> On Fri, Sep 06, 2013 at 03:59:02PM +0100, Stefano Stabellini wrote:
> > On Fri, 6 Sep 2013, Catalin Marinas wrote:
> > > On Thu, Sep 05, 2013 at 05:43:33PM +0100, Stefano Stabellini wrote:
> > > > On Thu, 5 Sep 2013, Catalin Marinas wrote:
> > > > > On Thu, Aug 29, 2013 at 07:32:32PM +0100, Stefano Stabellini wrote:
> > > > > > xen_swiotlb_alloc_coherent needs to allocate a coherent buffer for cpu
> > > > > > and devices. On native x86 and ARMv8 is sufficient to call
> > > > > > __get_free_pages in order to get a coherent buffer, while on ARM we need
> > > > > > to call arm_dma_ops.alloc.
> > > > > 
> > > > > Don't bet on this for ARMv8. It's not mandated for the architecture, so
> > > > > at some point some SoC will require non-cacheable buffers for coherency.
> > > > 
> > > > I see.
> > > > Would it be better if I implemented xen_alloc_coherent_pages on armv8 by
> > > > calling arm64_swiotlb_dma_ops.alloc?
> > > 
> > > What does this buffer do exactly? Is it allocated by guests?
> > 
> > It is allocated by Dom0 to do DMA to/from a device.
> > It is the buffer that is going to be returned by dma_map_ops.alloc to
> > the caller:
> > 
> > On x86:
> > dma_map_ops.alloc -> xen_swiotlb_alloc_coherent -> xen_alloc_coherent_pages -> __get_free_pages
> > 
> > On ARM:
> > dma_map_ops.alloc -> xen_swiotlb_alloc_coherent -> xen_alloc_coherent_pages -> arm_dma_ops.alloc
> > 
> > On ARM64
> > dma_map_ops.alloc -> xen_swiotlb_alloc_coherent -> xen_alloc_coherent_pages -> ????
> 
> OK, I'm getting more confused. Do all the above calls happen in the
> guest, Dom0, or a mix?

I guess the confusion comes from a difference in terminology: dom0 is a
guest like the others, just a bit more privileged. We usually call domU
a normal unprivileged guest.

The above calls would happen in Dom0 (when an SMMU is not available).
They could also happen in a DomU if we assign a physical device to it
(and an SMMU is not available).

So in general they would happen in any guest that needs to program a
real device.
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