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[85.223.209.55]) by smtp.gmail.com with ESMTPSA id d6-v6sm5794871ljd.52.2018.06.11.06.13.14 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Jun 2018 06:13:14 -0700 (PDT) Subject: Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA buffers To: Boris Ostrovsky , Stefano Stabellini , "Oleksandr_Andrushchenko@epam.com" Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org, jgross@suse.com, konrad.wilk@oracle.com, daniel.vetter@intel.com, matthew.d.roper@intel.com, dongwon.kim@intel.com, julien.grall@arm.com References: <20180601114132.22596-1-andr2000@gmail.com> <20180601114132.22596-6-andr2000@gmail.com> <64facf05-0a51-c3d9-9d3b-780893248628@oracle.com> <84217eac-b83b-710f-39ab-c93cad65bf9a@gmail.com> <30fa03c0-1b75-c0b1-b14f-8b52ea584e20@gmail.com> <78dc2fc4-cdac-05b7-2c34-22b69e7e009c@oracle.com> <4be24882-185d-01e3-6aa1-751e341433c7@gmail.com> From: Oleksandr Andrushchenko Message-ID: <06eff3fe-3ffc-47f6-6bd6-d8f2f823b382@gmail.com> Date: Mon, 11 Jun 2018 16:13:13 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/08/2018 10:21 PM, Boris Ostrovsky wrote: > On 06/08/2018 01:59 PM, Stefano Stabellini wrote: >>>>>>>>>    @@ -325,6 +401,14 @@ static int map_grant_pages(struct >>>>>>>>> grant_map >>>>>>>>> *map) >>>>>>>>>            map->unmap_ops[i].handle = map->map_ops[i].handle; >>>>>>>>>            if (use_ptemod) >>>>>>>>>                map->kunmap_ops[i].handle = >>>>>>>>> map->kmap_ops[i].handle; >>>>>>>>> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC >>>>>>>>> +        else if (map->dma_vaddr) { >>>>>>>>> +            unsigned long mfn; >>>>>>>>> + >>>>>>>>> +            mfn = __pfn_to_mfn(page_to_pfn(map->pages[i])); >>>>>>>> Not pfn_to_mfn()? >>>>>>> I'd love to, but pfn_to_mfn is only defined for x86, not ARM: [1] >>>>>>> and [2] >>>>>>> Thus, >>>>>>> >>>>>>> drivers/xen/gntdev.c:408:10: error: implicit declaration of function >>>>>>> ‘pfn_to_mfn’ [-Werror=implicit-function-declaration] >>>>>>>      mfn = pfn_to_mfn(page_to_pfn(map->pages[i])); >>>>>>> >>>>>>> So, I'll keep __pfn_to_mfn >>>>>> How will this work on non-PV x86? >>>>> So, you mean I need: >>>>> #ifdef CONFIG_X86 >>>>> mfn = pfn_to_mfn(page_to_pfn(map->pages[i])); >>>>> #else >>>>> mfn = __pfn_to_mfn(page_to_pfn(map->pages[i])); >>>>> #endif >>>>> >>>> I'd rather fix it in ARM code. Stefano, why does ARM uses the >>>> underscored version? >>> Do you want me to add one more patch for ARM to wrap __pfn_to_mfn >>> with static inline for ARM? e.g. >>> static inline ...pfn_to_mfn(...) >>> { >>>     __pfn_to_mfn(); >>> } >> A Xen on ARM guest doesn't actually know the mfns behind its own >> pseudo-physical pages. This is why we stopped using pfn_to_mfn and >> started using pfn_to_bfn instead, which will generally return "pfn", >> unless the page is a foreign grant. See include/xen/arm/page.h. >> pfn_to_bfn was also introduced on x86. For example, see the usage of >> pfn_to_bfn in drivers/xen/swiotlb-xen.c. Otherwise, if you don't care >> about other mapped grants, you can just use pfn_to_gfn, that always >> returns pfn. > > I think then this code needs to use pfn_to_bfn(). Ok > > >> Also, for your information, we support different page granularities in >> Linux as a Xen guest, see the comment at include/xen/arm/page.h: >> >> /* >> * The pseudo-physical frame (pfn) used in all the helpers is always based >> * on Xen page granularity (i.e 4KB). >> * >> * A Linux page may be split across multiple non-contiguous Xen page so we >> * have to keep track with frame based on 4KB page granularity. >> * >> * PV drivers should never make a direct usage of those helpers (particularly >> * pfn_to_gfn and gfn_to_pfn). >> */ >> >> A Linux page could be 64K, but a Xen page is always 4K. A granted page >> is also 4K. We have helpers to take into account the offsets to map >> multiple Xen grants in a single Linux page, see for example >> drivers/xen/grant-table.c:gnttab_foreach_grant. Most PV drivers have >> been converted to be able to work with 64K pages correctly, but if I >> remember correctly gntdev.c is the only remaining driver that doesn't >> support 64K pages yet, so you don't have to deal with it if you don't >> want to. > > I believe somewhere in this series there is a test for PAGE_SIZE vs. > XEN_PAGE_SIZE. Right, Oleksandr? Not in gntdev. You might have seen this in xen-drmfront/xen-sndfront, but I didn't touch gntdev for that. Do you want me to add yet another patch in the series to check for that? > Thanks for the explanation. > > -boris Thank you, Oleksandr