Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA buffers | From | Julien Grall <> | Date | Mon, 11 Jun 2018 18:46:45 +0100 |
| |
Hi,
On 06/11/2018 06:16 PM, Oleksandr Andrushchenko wrote: > On 06/11/2018 07:51 PM, Stefano Stabellini wrote: >> On Mon, 11 Jun 2018, Oleksandr Andrushchenko wrote: >>> 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? >> gntdev.c is already not capable of handling PAGE_SIZE != XEN_PAGE_SIZE, >> so you are not going to break anything that is not already broken :-) If >> your new gntdev.c code relies on PAGE_SIZE == XEN_PAGE_SIZE, it might be >> good to add an in-code comment about it, just to make it easier to fix >> the whole of gntdev.c in the future. >> > Yes, I just mean I can add something like [1] as a separate patch to the > series, > so we are on the safe side here
See my comment on Stefano's e-mail. I believe gntdev is able to handle PAGE_SIZE != XEN_PAGE_SIZE. So I would rather keep the behavior we have today for such case.
Cheers,
-- Julien Grall
| |