When tboot rewrites the e820 map on the X1 Carbon (booting Legacy, not
UEFI), it discards a >2GiB region of memory below the 4GiB boundary.
e820 map without tboot:
[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009cfff] usable
[ 0.000000] BIOS-e820: [mem 0x000000000009d000-0x000000000009ffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000000fffffff] usable
[ 0.000000] BIOS-e820: [mem 0x0000000010000000-0x000000001000afff]
reserved
[ 0.000000] BIOS-e820: [mem 0x000000001000b000-0x00000000bdcaffff] usable
[ 0.000000] BIOS-e820: [mem 0x00000000bdcb0000-0x00000000cc7fefff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000cc7ff000-0x00000000cc97efff] ACPI
NVS
[ 0.000000] BIOS-e820: [mem 0x00000000cc97f000-0x00000000cc9fefff] ACPI
data
[ 0.000000] BIOS-e820: [mem 0x00000000cc9ff000-0x00000000cfffffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000f8000000-0x00000000fbffffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fec00000-0x00000000fec00fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fed08000-0x00000000fed08fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fed10000-0x00000000fed19fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fed1c000-0x00000000fed1ffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fee00000-0x00000000fee00fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000ffa00000-0x00000000ffffffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000022dffffff] usable
e820 map with tboot:
[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000005ffff] usable
[ 0.000000] BIOS-e820: [mem 0x0000000000060000-0x0000000000067fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x0000000000068000-0x000000000009cfff] usable
[ 0.000000] BIOS-e820: [mem 0x000000000009d000-0x000000000009ffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x0000000000100000-0x00000000007fffff] usable
[ 0.000000] BIOS-e820: [mem 0x0000000000800000-0x0000000000ac8fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x0000000000ac9000-0x000000000fffffff] usable
[ 0.000000] BIOS-e820: [mem 0x0000000010000000-0x00000000cc7fefff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000cc7ff000-0x00000000cc97efff] ACPI
NVS
[ 0.000000] BIOS-e820: [mem 0x00000000cc97f000-0x00000000cc9fefff] ACPI
data
[ 0.000000] BIOS-e820: [mem 0x00000000cc9ff000-0x00000000cfffffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000f8000000-0x00000000fbffffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fec00000-0x00000000fec00fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fed08000-0x00000000fed08fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fed10000-0x00000000fed19fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fed1c000-0x00000000fed2ffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fee00000-0x00000000fee00fff]
reserved
[ 0.000000] BIOS-e820: [mem 0x00000000ffa00000-0x00000000ffffffff]
reserved
[ 0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000022dffffff] usable
Note the missing "[mem 0x000000001000b000-0x00000000bdcaffff] usable" line
when tboot is in use, represending ~2.6GiB.
Culprit code is common/e820.c:630-636. My interpretation of the comment
nearby is that this code is trying to avoid DMA-protecting reserved regions
<4GiB, but that region isn't reserved, nor does it overlap with any
reserved regions on this platform. I suspect this code is just being a
little bit too conservative.
Help?
Justin
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