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Date: Wed, 4 Mar 2015 21:17:48 +0100 From: Ingo Molnar <mingo@...nel.org> To: Toshi Kani <toshi.kani@...com> Cc: Andrew Morton <akpm@...ux-foundation.org>, "hpa@...or.com" <hpa@...or.com>, "tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "arnd@...db.de" <arnd@...db.de>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "x86@...nel.org" <x86@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "dave.hansen@...el.com" <dave.hansen@...el.com>, "Elliott, Robert (Server Storage)" <Elliott@...com> Subject: Re: [PATCH v3 6/6] x86, mm: Support huge KVA mappings on x86 * Toshi Kani <toshi.kani@...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@...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@...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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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