Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753030AbaKGSOk (ORCPT ); Fri, 7 Nov 2014 13:14:40 -0500 Received: from foss-mx-na.foss.arm.com ([217.140.108.86]:54791 "EHLO foss-mx-na.foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751278AbaKGSOi (ORCPT ); Fri, 7 Nov 2014 13:14:38 -0500 Date: Fri, 7 Nov 2014 18:14:30 +0000 From: Catalin Marinas To: Stefano Stabellini Cc: Will Deacon , "xen-devel@lists.xensource.com" , "konrad.wilk@oracle.com" , "Ian.Campbell@citrix.com" , "david.vrabel@citrix.com" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH v7 3/8] arm64: introduce is_device_dma_coherent Message-ID: <20141107181430.GH29148@e104818-lin.cambridge.arm.com> References: <20141105165646.GN32700@e104818-lin.cambridge.arm.com> <20141106103337.GA19702@e104818-lin.cambridge.arm.com> <20141107110524.GA21875@localhost> <20141107160006.GE29148@e104818-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 07, 2014 at 05:35:41PM +0000, Stefano Stabellini wrote: > On Fri, 7 Nov 2014, Stefano Stabellini wrote: > > On Fri, 7 Nov 2014, Catalin Marinas wrote: > > > What I would like to see is xen_dma_map_page() also using hyp calls for > > > cache maintenance when !pfn_valid(), for symmetry with unmap. You would > > > need another argument to xen_dma_map_page() though to pass the real > > > device address or mfn (and on the map side you could simply check for > > > page_to_pfn(page) != mfn). For such cases, Xen swiotlb already handles > > > bouncing so you don't need dom0 swiotlb involved as well. > > > > I can see that it would be nice to have map_page and unmap_page be > > symmetrical. However actually doing the map_page flush with an hypercall > > would slow things down. Hypercalls are slower than function calls. It is > > faster to do the cache flushing in dom0 if possible. In the map_page > > case we have the struct page so we can easily do it by calling the > > native dma_ops function. > > > > Maybe I could just add a comment to explain the reason for the asymmetry? > > Ah, but the problem is that map_page could allocate a swiotlb buffer > (actually it does on arm64) that without a corresponding unmap_page > call, would end up being leaked, right? Yes. You could hack dma_capable() to always return true for dom0 (because the pfn/dma address here doesn't have anything to do with the real mfn) but that's more of a hack assuming a lot about the swiotlb implementation. > Oh well.. two hypercalls it is then :-/ That looks cleaner to me (though I haven't seen the code yet ;)). Alternatively, you could keep a permanent page (per-cpu) in dom0 that you ask the hypervisor to point at the mfn just for the unmap cache maintenance (similarly to the kernel kmap_atomic used for cache maintenance on high pages). This could be more expensive but, OTOH, you only need the hyp call on the unmap path. The is_device_dma_coherent() would still remain to avoid any unnecessary map/unmap calls but for non-coherent devices you use whatever the SoC provides. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/