From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:47735) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RFQAK-0006Ot-Il for qemu-devel@nongnu.org; Sun, 16 Oct 2011 08:50:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1RFQAI-0006ON-5V for qemu-devel@nongnu.org; Sun, 16 Oct 2011 08:50:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:3430) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RFQAH-0006MP-SD for qemu-devel@nongnu.org; Sun, 16 Oct 2011 08:50:06 -0400 Date: Sun, 16 Oct 2011 14:50:56 +0200 From: "Michael S. Tsirkin" Message-ID: <20111016125054.GA17978@redhat.com> References: <1315197304-22469-2-git-send-email-david@gibson.dropbear.id.au> <20111002102547.GC30747@redhat.com> <4E883CF4.6060606@redhat.com> <20111002105238.GE30747@redhat.com> <4E8843DB.1020404@redhat.com> <20111002111700.GF30747@redhat.com> <4E885286.30905@redhat.com> <20111002121426.GK30747@redhat.com> <4E89B5D1.4080600@us.ibm.com> <20111014021407.GB4580@truffala.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111014021407.GB4580@truffala.fritz.box> Subject: Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori , Avi Kivity , qemu-devel@nongnu.org, joerg.roedel@amd.com, rth@twiddle.net, agraf@suse.de, eduard.munteanu@linux360.ro, kraxel@redhat.com On Fri, Oct 14, 2011 at 01:14:07PM +1100, David Gibson wrote: > On Mon, Oct 03, 2011 at 08:17:05AM -0500, Anthony Liguori wrote: > > On 10/02/2011 07:14 AM, Michael S. Tsirkin wrote: > > >On Sun, Oct 02, 2011 at 02:01:10PM +0200, Avi Kivity wrote: > > >>>Hmm, not entirely virtio specific, some devices use stX macros to do the > > >>>conversion. E.g. stw_be_phys and stl_le_phys are used in several > > >>>places. > > >> > > >>These are fine - explicit endianness. > > > > > >Right. So changing these to e.g. stl_dma and assuming > > >LE is default seems like a step backwards. > > > > We're generalizing too much. > > > > In general, the device model doesn't need atomic access functions. > > That's because device model RAM access is not coherent with CPU RAM > > access. > > Ok, so the next spin of these patches will have explicit LE and BE > versions of the accessors by popular demand. I'm still using > cpu_physical_memory_rw() as the backend though, because I can't see a > case where a device could safely _require_ an emulated DMA access to > be atomic. You don't? PCI spec supports atomic operations. It also strongly recommends not splitting accesses below dword boundary. > > Virtio is a very, very special case. virtio requires coherent RAM access. > > Right. Virtio's access to memory is *not* emulated PCI DMA, it's > god-like hypervisor access to guest system memory. It should > correctly bypass any IOMMU, and so should remain as > cpu_physical_memory_rw() or the atomic accessors, rather than being > converted to this new API. > > -- > David Gibson | I'll have my music baroque, and my code > david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ > | _way_ _around_! > http://www.ozlabs.org/~dgibson