Re: [Xen-devel] [PATCH v4 1/5] pci: add pci_iomap_wc() variants

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

On Thu, Apr 30, 2015 at 10:03:18AM -0700, Andy Lutomirski wrote:
> On Thu, Apr 30, 2015 at 9:52 AM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
> > On Thu, Apr 30, 2015 at 10:59:17AM -0500, Bjorn Helgaas wrote:
> >> [+cc linux-pci]
> >>
> >> Hi Luis,
> >>
> >> On Wed, Apr 29, 2015 at 02:36:08PM -0700, Luis R. Rodriguez wrote:
> >> > From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> >> >
> >> > This allows drivers to take advantage of write-combining
> >> > when possible. Ideally we'd have pci_read_bases() just
> >> > peg an IORESOURCE_WC flag for us
> >>
> >> This makes it sound like pci_read_bases() could do a better job
> >> if we just tried harder, but I don't think that's the case.  All
> >> pci_read_bases() can do is look at the bits in the BAR.  For
> >> memory BARs, there's a "prefetchable" bit and a "64-bit" bit.
> >>
> >> If you just want to complain that the PCI spec didn't define a
> >> way for software to discover whether a BAR can be mapped with WC,
> >> that's fine, but it's misleading to suggest that pci_read_bases()
> >> could figure out WC without some help from the spec.
> >
> > You're right sorry about that, in my original patch this was more
> > of a question and I did not have a full answer for but mst had
> > clarified before the spec doesn't allow for this [0] and you are
> > confirming this now as well.
> >
> > [0] https://lkml.org/lkml/2015/4/21/714
> >
> > I'll update the patch and at least document we did think about
> > this and that its a shortcoming of the spec.
> >
> >> > but where exactly
> >> > video devices memory lie varies *largely* and at times things
> >> > are mixed with MMIO registers, sometimes we can address
> >> > the changes in drivers, other times the change requires
> >> > intrusive changes.
> >> >
> >> > Although there is also arch_phys_wc_add() that makes use of
> >> > architecture specific write-combining alternatives (MTRR on
> >> > x86 when a system does not have PAT) we void polluting
> >> > pci_iomap() space with it and force drivers and subsystems
> >> > that want to use it to be explicit.
> >>
> >> I'm not quite sure I understand the point you're making here
> >> about not polluting pci_iomap_wc() with arch_phys_wc_add().  I
> >> think the choice is for a driver to do either this:
> >>
> >>   info->screen_base = pci_iomap_wc(dev, 0, 0);
> >>
> >> or this:
> >>
> >>   info->screen_base = pci_iomap_wc(dev, 0, 0);
> >>   par->wc_cookie = arch_phys_wc_add(pci_resource_start(dev, 0),
> >>                                   pci_resource_len(dev, 0));
> >>
> >> The driver is *already* being explicit because it calls
> >> pci_iomap_wc() instead of pci_iomap().
> >>
> >> It seems like it would be ideal if ioremap_wc() could call
> >> arch_phys_wc_add() internally.
> >
> > Indeed, that's what I was alluding to.
> >
> >> Doesn't any caller of
> >> arch_phys_wc_add() have to also do some sort of ioremap()
> >> beforehand?
> >
> > This is not a requirement as the physical address is used,
> > not the virtual address.
> >
> >> I assume there's some reason for separating them,
> >
> > Well a full sweep to change to arch_phys_wc_add() was never done,
> > consider this part of the last effort to do so. In retrospect now
> > that I've covered all other drivers in 12 different series of patches
> > I think its perhaps best to not mesh them together as we're phasing
> > out MTRR and the only reason to have arch_phys_wc_add() is for MTRR
> > which is legacy.
> 
> I would say it much more strongly.
> 
> Drivers for new hardware SHOULD NOT call arch_phys_wc_add, directly or
> otherwise.  MTRRs are crap.  They have nasty alignment requirements,
> they are a very limited and unpredictable resource, and the interact
> poorly with BIOS.  They should really only be used for old video
> framebuffers and such.
> 
> Anything new should use PAT (it's been available for a long time) and
> possibly streaming memory writes.  Even fancy server gear (myri10ge,
> for example) should stay far away from MTRRs and such: it's very easy
> to put enough devices in a server board that you simply run out of
> MTRRs and arch_phys_wc_add will stop working.
> 
> If we make ioremap_wc and similar start automatically adding MTRRs,
> then performance will vary wildly with the order of driver loading,
> because we'll run out of MTRRs part-way through bootup.
> 
> ioremap_wc via PAT, on the other hand, is 100% reliable on newer hardware.
> 
> Maybe I should have called it arch_phys_wc_add_awful_legacy_hack.

Thanks, I'll document such technicalities as well ;)

 Luis

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