6a79296cb1
Both the intent and the effect of reserve_bios_regions() is simple: reserve the range from the apparent BIOS start (suitably filtered) through 1MB and, if the EBDA start address is sensible, extend that reservation downward to cover the EBDA as well. The code is overcomplicated, though, and contains head-scratchers like: if (ebda_start < BIOS_START_MIN) ebda_start = BIOS_START_MAX; That snipped is trying to say "if ebda_start < BIOS_START_MIN, ignore it". Simplify it: reorder the code so that it makes sense. This should have no functional effect under any circumstances. Signed-off-by: Andy Lutomirski <luto@kernel.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Kees Cook <keescook@chromium.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Luis R. Rodriguez <mcgrof@suse.com> Cc: Mario Limonciello <mario_limonciello@dell.com> Cc: Matthew Garrett <mjg59@srcf.ucam.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Toshi Kani <toshi.kani@hp.com> Link: http://lkml.kernel.org/r/ef89c0c761be20ead8bd9a3275743e6259b6092a.1469135598.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
98 lines
3.1 KiB
C
98 lines
3.1 KiB
C
#include <linux/kernel.h>
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#include <linux/init.h>
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#include <linux/memblock.h>
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#include <asm/setup.h>
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#include <asm/bios_ebda.h>
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/*
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* This function reserves all conventional PC system BIOS related
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* firmware memory areas (some of which are data, some of which
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* are code), that must not be used by the kernel as available
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* RAM.
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*
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* The BIOS places the EBDA/XBDA at the top of conventional
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* memory, and usually decreases the reported amount of
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* conventional memory (int 0x12) too.
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*
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* This means that as a first approximation on most systems we can
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* guess the reserved BIOS area by looking at the low BIOS RAM size
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* value and assume that everything above that value (up to 1MB) is
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* reserved.
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*
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* But life in firmware country is not that simple:
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*
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* - This code also contains a quirk for Dell systems that neglect
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* to reserve the EBDA area in the 'RAM size' value ...
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*
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* - The same quirk also avoids a problem with the AMD768MPX
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* chipset: reserve a page before VGA to prevent PCI prefetch
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* into it (errata #56). (Usually the page is reserved anyways,
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* unless you have no PS/2 mouse plugged in.)
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*
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* - Plus paravirt systems don't have a reliable value in the
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* 'BIOS RAM size' pointer we can rely on, so we must quirk
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* them too.
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*
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* Due to those various problems this function is deliberately
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* very conservative and tries to err on the side of reserving
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* too much, to not risk reserving too little.
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*
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* Losing a small amount of memory in the bottom megabyte is
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* rarely a problem, as long as we have enough memory to install
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* the SMP bootup trampoline which *must* be in this area.
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*
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* Using memory that is in use by the BIOS or by some DMA device
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* the BIOS didn't shut down *is* a big problem to the kernel,
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* obviously.
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*/
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#define BIOS_RAM_SIZE_KB_PTR 0x413
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#define BIOS_START_MIN 0x20000U /* 128K, less than this is insane */
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#define BIOS_START_MAX 0x9f000U /* 640K, absolute maximum */
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void __init reserve_bios_regions(void)
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{
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unsigned int bios_start, ebda_start;
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/*
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* NOTE: In a paravirtual environment the BIOS reserved
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* area is absent. We'll just have to assume that the
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* paravirt case can handle memory setup correctly,
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* without our help.
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*/
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if (!x86_platform.legacy.reserve_bios_regions)
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return;
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/*
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* BIOS RAM size is encoded in kilobytes, convert it
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* to bytes to get a first guess at where the BIOS
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* firmware area starts:
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*/
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bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
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bios_start <<= 10;
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/*
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* If bios_start is less than 128K, assume it is bogus
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* and bump it up to 640K. Similarly, if bios_start is above 640K,
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* don't trust it.
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*/
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if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
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bios_start = BIOS_START_MAX;
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/* Get the start address of the EBDA page: */
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ebda_start = get_bios_ebda();
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/*
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* If the EBDA start address is sane and is below the BIOS region,
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* then also reserve everything from the EBDA start address up to
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* the BIOS region.
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*/
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if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)
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bios_start = ebda_start;
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/* Reserve all memory between bios_start and the 1MB mark: */
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memblock_reserve(bios_start, 0x100000 - bios_start);
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}
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