From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: [PATCH v4 10/10] xen/arm,arm64: enable SWIOTLB_XEN Date: Fri, 30 Aug 2013 09:22:10 -0400 Message-ID: <20130830132210.GE21239@phenom.dumpdata.com> References: <1376565054-24153-10-git-send-email-stefano.stabellini@eu.citrix.com> <1376600325.9273.223.camel@hastur.hellion.org.uk> <1377765650.11455.7.camel@kazak.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Stefano Stabellini Cc: Ian Campbell , xen-devel@lists.xensource.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org List-Id: xen-devel@lists.xenproject.org On Thu, Aug 29, 2013 at 04:38:17PM +0100, Stefano Stabellini wrote: > On Thu, 29 Aug 2013, Ian Campbell wrote: > > On Wed, 2013-08-28 at 21:07 +0100, Stefano Stabellini wrote: > > > On Thu, 15 Aug 2013, Ian Campbell wrote: > > > > On Thu, 2013-08-15 at 12:10 +0100, Stefano Stabellini wrote: > > > > > At the moment always rely on swiotlb-xen, but when Xen starts supporting > > > > > hardware IOMMUs we'll be able to avoid it conditionally on the presence > > > > > of an IOMMU on the platform. > > > > > > > > Do we have any idea how we are going to do this? > > > > > > > > It's extra complicated if you consider that on some systems on some of > > > > the devices are behind an IOMMU :-/ > > > > > > > > I wonder if we can enumerate which devices have an IOMMU at boot time > > > > and force a ludicrous dma mask (such as 0) if one isn't present in order > > > > to force us to always take the exchange_and_pin path? > > > > > > We don't need to worry about how to specify which devices need to go via > > > the swiotlb internally, because we have our own arm specific > > > dma_map_ops. At the moment they are just implemented using the > > > swiotlb-xen functions, but we could easily provide wrappers that check > > > our own internal whitelist/blacklist and go via swiotlb-xen only in > > > those cases. > > > > OK, but how do we decide which devices go on those lists? We need some > > sort of indication from the hypervisor, don't we? Only Xen knows if it > > has an iommu it can use because the iommu must necessarily be hidden > > from Linux. > > Right. Maybe Xen could mark the devices that are safe to use without > swiotlb-xen on device tree? Adding a property to the node? We do a hypercall for adding devices (See drivers/xen/pci.c). Perhaps you can piggyback on that one and on ARM return 0 for the devices that are under Xen control (so IOMMU) and for non-Xen control (those that will need SWIOTLB) return -EYOUNEEDTOHANDLEIT and use it?