Re: iwx 64-bit dma

Vitaliy Makkoveev <[email protected]> wrote:

> On Wed, Jul 29, 2026 at 07:12:17PM -0600, Theo de Raadt wrote:
> > Vitaliy Makkoveev <[email protected]> wrote:
> > 
> > > On Wed, Jul 29, 2026 at 06:51:49PM -0600, Theo de Raadt wrote:
> > > > Vitaliy Makkoveev <[email protected]> wrote:
> > > > 
> > > > > The IFXF_MBUF_64BIT flag affects only amd64. All other architectures
> > > > > still uses memory below 4G. See revision 1.61 of
> > > > > sys/arch/amd64/amd64/autoconf.c line 129.
> > > > 
> > > > I don't see how that matters, that's part of the mechanism for
> > > > providing memory to drivers.
> > > > 
> > > > The diff in question which adds BUS_DMA_64BIT is a declaration to
> > > > the higher-level code that _all_ chipsets supported by this driver can
> > > > work with 64 bit-addressable memory if the higher levels provide it.
> > > > 
> > > > If one chipset can't do it, the experience will be terrible.
> > > > 
> > > > 
> > > > 
> > > 
> > > Well, this means the revision 1.198 of sys/dev/pci/if_iwx.c is wrong and
> > > should be reverted.
> > 
> > No.
> > 
> > It might answer the question I was asking.
> 
> I don't understand your objections.

I did not object.

> The commit message explicitly says
> "this hardware has no issues handling full 64-bit addresses" and the
> commit enables 64 bits DMA transfers for *all* iwx(4) chipsets. This
> means it can access memory above 4G, right? If so, nothing stops us to
> set BUS_DMA_64BIT and allocate mbufs from high memory.

What commit message says that?  Something a few year ago?

I asked a question.

It didn't get answered directly instead there's an assumption that I
should have trawled through the tree looking for the answer.

> Your suggestion is that some hypothetical iwx(4) chipset does not
> support 64 bits DMA transfers.

No.  I said it is important to verify this.  Has it now been verified?
YES.  Your sentence makes no sense.

> OK, in such case we should follow em(4)
> or re(4) way and enable 64 bits DMA transfers *only for* capable
> devices:
> 
>         if (sc->hw.bus_type == em_bus_type_pci_express)
>                 sc->sc_dmaflags |= BUS_DMA_64BIT;
> 	/* ... */
> 	r = bus_dmamap_create(sc->sc_dmat, ..., sc->sc_dmaflags, ...);
> 	/* ... */
>         if (ISSET(sc->sc_dmaflags, BUS_DMA_64BIT))
>                 ifp->if_xflags |= IFXF_MBUF_64BIT;

Yes, that is what we should do if that situation arises.

> So the current sys/dev/pci/if_iwx.c which enables 64 bits DMA transfers
> for all iwx(4) devices, but lefts mbufs allocation from below 4G is
> wrong. What is the reason for this BUS_DMA_64BIT flag on iwx(4) if we
> don't allow device to access high memory? Why iwx(4) should be special
> one? Why don't we follow this way on all devices?

I asked "Are we sure all the iwx devices can do this".

And I will continue to ask questions like this EVERYTIME someone sends a
diff which comes with text like:

     "This change works for my one device"