On Wed, 2015-03-04 at 21:17 +0100, Ingo Molnar wrote:
> * Toshi Kani <toshi.kani@xxxxxx> wrote:
>
> > On Wed, 2015-03-04 at 01:00 +0000, Andrew Morton wrote:
> > > On Tue, 03 Mar 2015 16:14:32 -0700 Toshi Kani <toshi.kani@xxxxxx> wrote:
> > >
> > > > On Tue, 2015-03-03 at 14:44 -0800, Andrew Morton wrote:
> > > > > On Tue, 3 Mar 2015 10:44:24 -0700 Toshi Kani <toshi.kani@xxxxxx> wrote:
> > > > :
> > > > > > +
> > > > > > +#ifdef CONFIG_HAVE_ARCH_HUGE_VMAP
> > > > > > +int pud_set_huge(pud_t *pud, phys_addr_t addr, pgprot_t prot)
> > > > > > +{
> > > > > > + u8 mtrr;
> > > > > > +
> > > > > > + /*
> > > > > > + * Do not use a huge page when the range is covered by non-WB type
> > > > > > + * of MTRRs.
> > > > > > + */
> > > > > > + mtrr = mtrr_type_lookup(addr, addr + PUD_SIZE);
> > > > > > + if ((mtrr != MTRR_TYPE_WRBACK) && (mtrr != 0xFF))
> > > > > > + return 0;
> > > > >
> > > > > It would be good to notify the operator in some way when this happens.
> > > > > Otherwise the kernel will run more slowly and there's no way of knowing
> > > > > why. I guess slap a pr_info() in there. Or maybe pr_warn()?
> > > >
> > > > We only use 4KB mappings today, so this case will not make it run
> > > > slowly, i.e. it will be the same as today.
> > >
> > > Yes, but it would be slower than it would be if the operator fixed the
> > > mtrr settings! How do we let the operator know this?
> > >
> > > > Also, adding a message here
> > > > can generate a lot of messages when MTRRs cover a large area.
> > >
> > > Really? This is only going to happen when a device driver
> > > requests a huge io mapping, isn't it? That's rare. We could emit
> > > a warning, return an error code and fall all the way back to the
> > > top-level ioremap code which can then retry with 4k mappings. Or
> > > something similar - somehow record the fact that this warning has
> > > been emitted or use printk ratelimiting (bad option).
> >
> > Yes, an IO device with a huge MMIO space that is covered by MTRRs is
> > a rare case. BIOS does not need to specify how MMIO of each card
> > needs to be accessed with MTRRs (or BIOS should not do it since an
> > MMIO address is configurable on each card).
> >
> > However, PCIe has the MMCONFIG space, PCIe config space, which is
> > also memory mapped and must be accessed with UC. The PCI subsystem
> > calls ioremap_nocache() to map the entire MMCONFIG space, which
> > covers the PCIe config space of all possible cards. Here are boot
> > messages on my test system.
> >
> > :
> > PCI: MMCONFIG for domain 0000 [bus 00-ff] at [mem 0xc0000000-0xcf
> > ffffff] (base 0xc0000000)
> > PCI: MMCONFIG at [mem 0xc0000000-0xcfffffff] reserved in E820
> > :
> >
> > And MTRRs cover this MMCONFIG space with UC to assure that the range is
> > always accessed with UC.
>
> So the PCI code ioremap()s this 256 MB mmconfig space in its entirety
> currently?
Yes.
> > # cat /proc/mtrr
> > reg00: base=0x0c0000000 ( 3072MB), size= 1024MB, count=1: uncachable
> >
> > So, if we add a message into the code, it will be displayed many
> > times in this ioremap_nocache() call from PCI.
>
> So, in this specific case, when a single MTRR covers it with a single
> cache policy, I think we can safely map it UC using hugepmds?
Yes.
> That will 'shut up' the warning the right way: by making the code
> work?
I see your point. I will look into mtrr_type_lookup() to see if we can
make it work in a manageable way.
Thanks,
-Toshi
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>