Commit edce2121 authored by Ingo Molnar's avatar Ingo Molnar

x86/boot: Reorganize and clean up the BIOS area reservation code

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's avatarIngo Molnar <mingo@kernel.org>
parent 4ff53087
......@@ -17,7 +17,7 @@ static inline unsigned int get_bios_ebda(void)
return address; /* 0 means none */
}
void reserve_ebda_region(void);
void reserve_bios_regions(void);
#ifdef CONFIG_X86_CHECK_BIOS_CORRUPTION
/*
......
......@@ -168,14 +168,14 @@ struct x86_legacy_devices {
* struct x86_legacy_features - legacy x86 features
*
* @rtc: this device has a CMOS real-time clock present
* @ebda_search: it's safe to search for the EBDA signature in the hardware's
* @reserve_bios_regions: it's safe to search for the EBDA signature in the hardware's
* low RAM
* @devices: legacy x86 devices, refer to struct x86_legacy_devices
* documentation for further details.
*/
struct x86_legacy_features {
int rtc;
int ebda_search;
int reserve_bios_regions;
struct x86_legacy_devices devices;
};
......
......@@ -6,66 +6,104 @@
#include <asm/bios_ebda.h>
/*
* This function reserves all conventional PC system BIOS related
* firmware memory areas (some of which are data, some of which
* are code), that must not be used by the kernel as available
* RAM.
*
* The BIOS places the EBDA/XBDA at the top of conventional
* memory, and usually decreases the reported amount of
* conventional memory (int 0x12) too. This also contains a
* workaround for Dell systems that neglect to reserve EBDA.
* The same workaround also avoids a problem with the AMD768MPX
* chipset: reserve a page before VGA to prevent PCI prefetch
* into it (errata #56). Usually the page is reserved anyways,
* unless you have no PS/2 mouse plugged in.
* conventional memory (int 0x12) too.
*
* This means that as a first approximation on most systems we can
* guess the reserved BIOS area by looking at the low BIOS RAM size
* value and assume that everything above that value (up to 1MB) is
* reserved.
*
* But life in firmware country is not that simple:
*
* - This code also contains a quirk for Dell systems that neglect
* to reserve the EBDA area in the 'RAM size' value ...
*
* - The same quirk also avoids a problem with the AMD768MPX
* chipset: reserve a page before VGA to prevent PCI prefetch
* into it (errata #56). (Usually the page is reserved anyways,
* unless you have no PS/2 mouse plugged in.)
*
* - Plus paravirt systems don't have a reliable value in the
* 'BIOS RAM size' pointer we can rely on, so we must quirk
* them too.
*
* Due to those various problems this function is deliberately
* very conservative and tries to err on the side of reserving
* too much, to not risk reserving too little.
*
* Losing a small amount of memory in the bottom megabyte is
* rarely a problem, as long as we have enough memory to install
* the SMP bootup trampoline which *must* be in this area.
*
* This functions is deliberately very conservative. Losing
* memory in the bottom megabyte is rarely a problem, as long
* as we have enough memory to install the trampoline. Using
* memory that is in use by the BIOS or by some DMA device
* the BIOS didn't shut down *is* a big problem.
* Using memory that is in use by the BIOS or by some DMA device
* the BIOS didn't shut down *is* a big problem to the kernel,
* obviously.
*/
#define BIOS_LOWMEM_KILOBYTES 0x413
#define LOWMEM_CAP 0x9f000U /* Absolute maximum */
#define INSANE_CUTOFF 0x20000U /* Less than this = insane */
#define BIOS_RAM_SIZE_KB_PTR 0x413
void __init reserve_ebda_region(void)
#define BIOS_START_MIN 0x20000U /* 128K, less than this is insane */
#define BIOS_START_MAX 0x9f000U /* 640K, absolute maximum */
void __init reserve_bios_regions(void)
{
unsigned int lowmem, ebda_addr;
unsigned int bios_start, ebda_start;
/*
* To determine the position of the EBDA and the
* end of conventional memory, we need to look at
* the BIOS data area. In a paravirtual environment
* that area is absent. We'll just have to assume
* that the paravirt case can handle memory setup
* correctly, without our help.
* NOTE: In a paravirtual environment the BIOS reserved
* area is absent. We'll just have to assume that the
* paravirt case can handle memory setup correctly,
* without our help.
*/
if (!x86_platform.legacy.ebda_search)
if (!x86_platform.legacy.reserve_bios_regions)
return;
/* end of low (conventional) memory */
lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
lowmem <<= 10;
/* start of EBDA area */
ebda_addr = get_bios_ebda();
/* Get the start address of the EBDA page: */
ebda_start = get_bios_ebda();
/*
* Note: some old Dells seem to need 4k EBDA without
* reporting so, so just consider the memory above 0x9f000
* to be off limits (bugzilla 2990).
* Quirk: some old Dells seem to have a 4k EBDA without
* reporting so in their BIOS RAM size value, so just
* consider the memory above 640K to be off limits
* (bugzilla 2990).
*
* We detect this case by filtering for nonsensical EBDA
* addresses below 128K, where we can assume that they
* are bogus and bump it up to a fixed 640K value:
*/
if (ebda_start < BIOS_START_MIN)
ebda_start = BIOS_START_MAX;
/* If the EBDA address is below 128K, assume it is bogus */
if (ebda_addr < INSANE_CUTOFF)
ebda_addr = LOWMEM_CAP;
/*
* BIOS RAM size is encoded in kilobytes, convert it
* to bytes to get a first guess at where the BIOS
* firmware area starts:
*/
bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
bios_start <<= 10;
/* If lowmem is less than 128K, assume it is bogus */
if (lowmem < INSANE_CUTOFF)
lowmem = LOWMEM_CAP;
/*
* If bios_start is less than 128K, assume it is bogus
* and bump it up to 640K:
*/
if (bios_start < BIOS_START_MIN)
bios_start = BIOS_START_MAX;
/* Use the lower of the lowmem and EBDA markers as the cutoff */
lowmem = min(lowmem, ebda_addr);
lowmem = min(lowmem, LOWMEM_CAP); /* Absolute cap */
/*
* Use the lower of the bios_start and ebda_start
* as the starting point, but don't allow it to
* go beyond 640K:
*/
bios_start = min(bios_start, ebda_start);
bios_start = min(bios_start, BIOS_START_MAX);
/* reserve all memory between lowmem and the 1MB mark */
memblock_reserve(lowmem, 0x100000 - lowmem);
/* Reserve all memory between bios_start and the 1MB mark: */
memblock_reserve(bios_start, 0x100000 - bios_start);
}
......@@ -26,7 +26,7 @@ static void __init i386_default_early_setup(void)
x86_init.resources.reserve_resources = i386_reserve_resources;
x86_init.mpparse.setup_ioapic_ids = setup_ioapic_ids_from_mpc;
reserve_ebda_region();
reserve_bios_regions();
}
asmlinkage __visible void __init i386_start_kernel(void)
......
......@@ -183,7 +183,7 @@ void __init x86_64_start_reservations(char *real_mode_data)
copy_bootdata(__va(real_mode_data));
x86_early_init_platform_quirks();
reserve_ebda_region();
reserve_bios_regions();
switch (boot_params.hdr.hardware_subarch) {
case X86_SUBARCH_INTEL_MID:
......
......@@ -7,12 +7,12 @@
void __init x86_early_init_platform_quirks(void)
{
x86_platform.legacy.rtc = 1;
x86_platform.legacy.ebda_search = 0;
x86_platform.legacy.reserve_bios_regions = 0;
x86_platform.legacy.devices.pnpbios = 1;
switch (boot_params.hdr.hardware_subarch) {
case X86_SUBARCH_PC:
x86_platform.legacy.ebda_search = 1;
x86_platform.legacy.reserve_bios_regions = 1;
break;
case X86_SUBARCH_XEN:
case X86_SUBARCH_LGUEST:
......
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