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x86: Add MPTABLE_LINUX_BUG_COMPAT option

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Linux has two bugs in its handling of the x86 MP table:
1. It assumes that there is always 640 kB of base memory, and looks for
the MP table in the top kB of this even if the memory map indicates
that memory location does not exist.
2. It ignores that entry_count field and instead iterates through the
MP table by scanning until it runs out of bytes in the table.

The Firecracker VM (and probably other related VMs) relies on both of
these bugs.  With the MPTABLE_LINUX_BUG_COMPAT option, we search for
the MP table at address 639k even if that isn't in the memory map; and
replace a zeroed entry_count with a value computed from scanning the
table until we run out of table bytes.

Reviewed by:	imp
Sponsored by:	https://www.patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D35799
This commit is contained in:
Colin Percival 2022-07-12 17:45:18 -07:00
parent 2297a1633d
commit d1ca8cc638
3 changed files with 56 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
sys/conf/options.amd64
View File

@ -63,3 +63,6 @@ EFIRT opt_efirt.h
# Don't create a "legacy" PCI bridge if none is found.
NO_LEGACY_PCIB opt_cpu.h
# Compatibility with Linux MP table bugs.
MPTABLE_LINUX_BUG_COMPAT

3
sys/conf/options.i386
View File

@ -107,3 +107,6 @@ ISCI_LOGGING opt_isci.h
# Don't create a "legacy" PCI bridge if none is found.
NO_LEGACY_PCIB opt_cpu.h
# Compatibility with Linux MP table bugs.
MPTABLE_LINUX_BUG_COMPAT

50
sys/x86/x86/mptable.c
View File

@ -30,6 +30,7 @@
__FBSDID("$FreeBSD$");
#include "opt_mptable_force_htt.h"
#include "opt_mptable_linux_bug_compat.h"
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/bus.h>
@ -245,6 +246,34 @@ lookup_bus_type(char *name)
return (UNKNOWN_BUSTYPE);
}
#ifdef MPTABLE_LINUX_BUG_COMPAT
/* Compute the correct entry_count value. */
static void
compute_entry_count(void)
{
u_char *end = (u_char *)(mpct) + mpct->base_table_length;
u_char *entry = (u_char *)(mpct + 1);
size_t nentries = 0;
while (entry < end) {
switch (*entry) {
case MPCT_ENTRY_PROCESSOR:
case MPCT_ENTRY_IOAPIC:
case MPCT_ENTRY_BUS:
case MPCT_ENTRY_INT:
case MPCT_ENTRY_LOCAL_INT:
break;
default:
panic("%s: Unknown MP Config Entry %d\n", __func__,
(int)*entry);
}
entry += basetable_entry_types[*entry].length;
nentries++;
}
mpct->entry_count = (uint16_t)(nentries);
}
#endif
/*
* Look for an Intel MP spec table (ie, SMP capable hardware).
*/
@ -273,6 +302,17 @@ mptable_probe(void)
if ((x = search_for_sig(target, BIOS_COUNT)) >= 0)
goto found;
#ifdef MPTABLE_LINUX_BUG_COMPAT
/*
* Linux assumes that it always has 640 kB of base memory and
* searches for the MP table at 639k regardless of whether that
* address is present in the system memory map. Some VM systems
* rely on this buggy behaviour.
*/
if ((x = search_for_sig(639 * 1024, 1024 / 4)) >= 0)
goto found;
#endif
/* nothing found */
return (ENXIO);
@ -321,6 +361,16 @@ found:
printf(
"MP Configuration Table version 1.%d found at %p\n",
mpct->spec_rev, mpct);
#ifdef MPTABLE_LINUX_BUG_COMPAT
/*
* Linux ignores entry_count and instead scans the MP table
* until it runs out of bytes of table (as specified by the
* base_table_length field). Some VM systems rely on this
* buggy behaviour and record an entry_count of zero.
*/
if (mpct->entry_count == 0)
compute_entry_count();
#endif
}
return (-100);