Re: [PATCH v3 6/6] x86, mm: Support huge KVA mappings on x86

[Ingo Molnar]

* 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?

> 
> # 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?

That will 'shut up' the warning the right way: by making the code 
work?

Thanks,

	Ingo
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