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

[Oleksandr Andrushchenko]

On 06/08/2018 10:21 PM, Boris Ostrovsky wrote:
> On 06/08/2018 01:59 PM, Stefano Stabellini wrote:
> > > > > > > > >   ÂÂ @@ -325,6 +401,14 @@ static int map_grant_pages(struct
> > > > > > > > > grant_map
> > > > > > > > > *map)
> > > > > > > > >   ÂÂÂÂÂÂÂÂÂÂ map->unmap_ops[i].handle = map->map_ops[i].handle;
> > > > > > > > >   ÂÂÂÂÂÂÂÂÂÂ if (use_ptemod)
> > > > > > > > >   ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ map->kunmap_ops[i].handle =
> > > > > > > > > map->kmap_ops[i].handle;
> > > > > > > > > +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
> > > > > > > > > +ÂÂÂÂÂÂÂ else if (map->dma_vaddr) {
> > > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂ unsigned long mfn;
> > > > > > > > > +
> > > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂ 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
> > > > > > > âpfn_to_mfnâ [-Werror=implicit-function-declaration]
> > > > > > >   ÂÂÂÂ 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(...)
> > > {
> > >  ÂÂÂ __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