Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA buffers

Stefano Stabellini
2018-06-11T16:51:18Z

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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.

> > Thanks for the explanation.

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