netdev - Re: [PATCH v8 10/15] octeontx2-af: Reconfig MSIX base with IOVA

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Date:   Tue, 9 Oct 2018 14:00:04 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Sunil Kovvuri <sunil.kovvuri@...il.com>
Cc:     Networking <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, linux-soc@...r.kernel.org,
        gakula@...vell.com, sgoutham@...vell.com
Subject: Re: [PATCH v8 10/15] octeontx2-af: Reconfig MSIX base with IOVA

On Tue, Oct 9, 2018 at 11:20 AM Sunil Kovvuri <sunil.kovvuri@...il.com> wrote:
>
> On Tue, Oct 9, 2018 at 1:27 PM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Tue, Oct 9, 2018 at 9:03 AM Sunil Kovvuri <sunil.kovvuri@...il.com> wrote:
> > > On Mon, Oct 8, 2018 at 5:38 PM Arnd Bergmann <arnd@...db.de> wrote:
> > > > On Sun, Oct 7, 2018 at 5:01 PM <sunil.kovvuri@...il.com> wrote:
> > I think if you enable CONFIG_DEBUG_VIRTUAL, the virt_to_page()
> > above should trigger a warning in
> >
> > phys_addr_t __virt_to_phys(unsigned long x)
> > {
> >         WARN(!__is_lm_address(x),
> >              "virt_to_phys used for non-linear address: %pK (%pS)\n",
> >               (void *)x,
> >               (void *)x);
> >
> >         return __virt_to_phys_nodebug(x);
> > }
> >
> > Can you verify that?
> >
> >         Arnd
>
> No, it isn't, as for 64bit systems CONFIG_SPARSEMEM_VMEMMAP is enabled.

But that is also user-selectable, right? It still seems to be really
fragile to rely on non-documented behavior of virt_to_phys()
here, if that only works for some configurations.

> But is there any alternative to what is being done ?

I'm not completely sure, but there may be a way to do this correctly
using the iommu API instead of the dma-mapping API. What you do
here is fairly rare, but not unprecedented.

      Arnd