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Date: Tue, 25 Jan 2011 09:29:58 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: "Ahmed S. Darwish" <darwish.07@gmail.com>
CC: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        X86-ML <x86@kernel.org>, Tony Luck <tony.luck@intel.com>,
        Dave Jones <davej@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Randy Dunlap <rdunlap@xenotime.net>,
        Willy Tarreau <wtarreau@hera.kernel.org>, Willy Tarreau <w@1wt.eu>,
        Dirk Hohndel <hohndel@infradead.org>, Dirk.Hohndel@intel.com,
        Simon Kagstrom <simon.kagstrom@netinsight.net>,
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        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -next 2/2][RFC] x86: Saveoops: Reserve low memory and
 register code
References: <20110125134748.GA10051@laptop> <20110125135326.GC10051@laptop>
In-Reply-To: <20110125135326.GC10051@laptop>
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On 01/25/2011 05:53 AM, Ahmed S. Darwish wrote:
> +
> +/*
> + * Extended BIOS services write to disk in units of 512-byte sectors.
> + * Thus, always align the ring buffer size on a 512-byte boundary.
> + */

Units of sectors, not always 512 bytes.  This needs to be done
correctly, or you will destroy real data.

> +/*
> + * Page tables to identity map the first 2 Mbytes.
> + */
> +static __aligned(PAGE_SIZE) pud_t ident_level3[PTRS_PER_PUD];
> +static __aligned(PAGE_SIZE) pmd_t ident_level2[PTRS_PER_PMD];
> +
> +/*
> + * The lmode->rmode switching code needs to run from an identity page
> + * since it disables paging.
> + */
> +static void build_identity_mappings(void)
> +{
> +	pgd_t *pgde;
> +	pud_t *pude;
> +	pmd_t *pmde;
> +
> +	pmde = ident_level2;
> +	set_pmd(pmde, __pmd(0 + __PAGE_KERNEL_IDENT_LARGE_EXEC));
> +
> +	pude = ident_level3;
> +	set_pud(pude, __pud(__pa(ident_level2) + _KERNPG_TABLE));
> +
> +	pgde = init_level4_pgt;
> +	set_pgd(pgde, __pgd(__pa(ident_level3) + _KERNPG_TABLE));
> +
> +	__flush_tlb_all();
> +}

We now have a permanent identity map so there is no point in building a
new one.

However, I'm quite nervous about this -- this patch has *plenty* of real
possibility of wrecking data.

	-hpa
-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.