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From: Ingo Molnar <mingo@kernel.org>
To: Toshi Kani <toshi.kani@hp.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"arnd@arndb.de" <arnd@arndb.de>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"dave.hansen@intel.com" <dave.hansen@intel.com>,
	"Elliott, Robert (Server Storage)" <Elliott@hp.com>
Subject: Re: [PATCH v3 6/6] x86, mm: Support huge KVA mappings on x86
Date: Wed, 4 Mar 2015 21:17:48 +0100	[thread overview]
Message-ID: <20150304201748.GA6634@gmail.com> (raw)
In-Reply-To: <1425486216.17007.236.camel@misato.fc.hp.com>


* Toshi Kani <toshi.kani@hp.com> 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@hp.com> 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@hp.com> 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

  reply	other threads:[~2015-03-04 20:17 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-03 17:44 [PATCH v3 0/6] Kernel huge I/O mapping support Toshi Kani
2015-03-03 17:44 ` [PATCH v3 1/6] mm: Change __get_vm_area_node() to use fls_long() Toshi Kani
2015-03-03 17:44 ` [PATCH v3 2/6] lib: Add huge I/O map capability interfaces Toshi Kani
2015-03-03 17:44 ` [PATCH v3 3/6] mm: Change ioremap to set up huge I/O mappings Toshi Kani
2015-03-04 22:09   ` Ingo Molnar
2015-03-04 23:15     ` Toshi Kani
2015-03-03 17:44 ` [PATCH v3 4/6] mm: Change vunmap to tear down huge KVA mappings Toshi Kani
2015-03-03 17:44 ` [PATCH v3 5/6] x86, mm: Support huge I/O mapping capability I/F Toshi Kani
2015-03-03 17:44 ` [PATCH v3 6/6] x86, mm: Support huge KVA mappings on x86 Toshi Kani
2015-03-03 22:44   ` Andrew Morton
2015-03-03 23:14     ` Toshi Kani
2015-03-04  1:00       ` Andrew Morton
2015-03-04 16:23         ` Toshi Kani
2015-03-04 20:17           ` Ingo Molnar [this message]
2015-03-04 21:16             ` Toshi Kani

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