From mboxrd@z Thu Jan 1 00:00:00 1970 From: Julien Grall Subject: Re: [PATCH v02 1/7] arm: introduce remoteprocessor iommu module Date: Tue, 22 Jul 2014 14:29:19 +0100 Message-ID: <53CE672F.9050501@linaro.org> References: <1403780826-22123-1-git-send-email-andrii.tseglytskyi@globallogic.com> <1403780826-22123-2-git-send-email-andrii.tseglytskyi@globallogic.com> <1405523940.1087.19.camel@kazak.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Stefano Stabellini , Ian Campbell Cc: Julien Grall , xen-devel@lists.xen.org, Andrii Tseglytskyi List-Id: xen-devel@lists.xenproject.org On 07/22/2014 01:42 PM, Stefano Stabellini wrote: > On Wed, 16 Jul 2014, Ian Campbell wrote: >> On Fri, 2014-07-04 at 14:59 +0100, Stefano Stabellini wrote: >>> On Thu, 26 Jun 2014, Andrii Tseglytskyi wrote: >>>> This is a fisrst patch from patch series which was >>>> developed to handle remote (external) processors >>>> memory management units. Remote processors are >>>> typically used for graphic rendering (GPUs) and >>>> high quality video decoding (IPUs). They are typically >>>> installed on such multimedia SoCs as OMAP4 / OMAP5. >>>> >>>> As soon as remoteprocessor MMU typically works with >>>> pagetables filled by physical addresses, which are >>>> allocated by domU kernel, it is almost impossible to >>>> use them under Xen, intermediate physical addresses >>>> allocated by kernel, need to be translated to machine >>>> addresses. >>>> >>>> This patch introduces a simple framework to perform >>>> pfn -> mfn translation for external MMUs. >>>> It introduces basic data structures and algorithms >>>> needed for translation. >>>> >>>> Typically, when MMU is configured, some it registers >>>> are updated by new values. Introduced frameworks >>>> uses traps as starting point of remoteproc MMUs >>>> pagetables translation. >>>> >>>> Change-Id: Ia4d311a40289df46a003f5ae8706c150bee1885d >>>> Signed-off-by: Andrii Tseglytskyi >>> >>> There is one problem with this patch: you need to find a way to "pin" >>> the p2m entries for the pfns and mfns found in the pagetables translated >>> by remoteproc_iommu.c. Otherwise Xen might decide to change the pfn to >>> mfn mappings for the domain, breaking the pagetables already translated >>> by remoteproc_iommu.c. pfn to mfn mappings are not guaranteed to be >>> stable. At the moment Xen on ARM wouldn't change pfn to mfn mappings >>> under the guest feet, but features like memory sharing or swapping, >>> supported on x86, could cause it to happen. >> >> I've not fully grokked this patch but wouldn't it be easier and better >> to have a hook to cause the remoteproc to throw away and rebuild its >> mappings when this happens? It could be called from e.g. the TLB flush >> associated with the p2m changing, I think. > > I guess it depends on how often Xen or the Xen tools are going to change > the p2m for a running domain. For example changing one page mapping a > couple of times a second would be very expensive if we have to rebuild > the remoteproc pagetables every time. The kernel could also change the p2m mapping via the ballooning driver. Regards, -- Julien Grall