edce21216a
So the reserve_ebda_region() code has accumulated a number of problems over the years that make it really difficult to read and understand: - The calculation of 'lowmem' and 'ebda_addr' is an unnecessarily interleaved mess of first lowmem, then ebda_addr, then lowmem tweaks... - 'lowmem' here means 'super low mem' - i.e. 16-bit addressable memory. In other parts of the x86 code 'lowmem' means 32-bit addressable memory... This makes it super confusing to read. - It does not help at all that we have various memory range markers, half of which are 'start of range', half of which are 'end of range' - but this crucial property is not obvious in the naming at all ... gave me a headache trying to understand all this. - Also, the 'ebda_addr' name sucks: it highlights that it's an address (which is obvious, all values here are addresses!), while it does not highlight that it's the _start_ of the EBDA region ... - 'BIOS_LOWMEM_KILOBYTES' says a lot of things, except that this is the only value that is a pointer to a value, not a memory range address! - The function name itself is a misnomer: it says 'reserve_ebda_region()' while its main purpose is to reserve all the firmware ROM typically between 640K and 1MB, while the 'EBDA' part is only a small part of that ... - Likewise, the paravirt quirk flag name 'ebda_search' is misleading as well: this too should be about whether to reserve firmware areas in the paravirt case. - In fact thinking about this as 'end of RAM' is confusing: what this function *really* wants to reserve is firmware data and code areas! Once the thinking is inverted from a mixed 'ram' and 'reserved firmware area' notion to a pure 'reserved area' notion everything becomes a lot clearer. To improve all this rewrite the whole code (without changing the logic): - Firstly invert the naming from 'lowmem end' to 'BIOS reserved area start' and propagate this concept through all the variable names and constants. BIOS_RAM_SIZE_KB_PTR // was: BIOS_LOWMEM_KILOBYTES BIOS_START_MIN // was: INSANE_CUTOFF ebda_start // was: ebda_addr bios_start // was: lowmem BIOS_START_MAX // was: LOWMEM_CAP - Then clean up the name of the function itself by renaming it to reserve_bios_regions() and renaming the ::ebda_search paravirt flag to ::reserve_bios_regions. - Fix up all the comments (fix typos), harmonize and simplify their formulation and remove comments that become unnecessary due to the much better naming all around. Signed-off-by: Ingo Molnar <mingo@kernel.org>
110 lines
3.4 KiB
C
110 lines
3.4 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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/* 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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* Quirk: some old Dells seem to have a 4k EBDA without
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* reporting so in their BIOS RAM size value, so just
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* consider the memory above 640K to be off limits
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* (bugzilla 2990).
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*
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* We detect this case by filtering for nonsensical EBDA
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* addresses below 128K, where we can assume that they
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* are bogus and bump it up to a fixed 640K value:
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*/
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if (ebda_start < BIOS_START_MIN)
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ebda_start = BIOS_START_MAX;
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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:
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*/
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if (bios_start < BIOS_START_MIN)
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bios_start = BIOS_START_MAX;
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/*
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* Use the lower of the bios_start and ebda_start
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* as the starting point, but don't allow it to
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* go beyond 640K:
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*/
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bios_start = min(bios_start, ebda_start);
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bios_start = min(bios_start, BIOS_START_MAX);
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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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