Why must we manually configure a serial console?
I was looking at ACPI tables in a bhyve guest and stumbled across this:
root@test:/tmp# acpidump -b spcr
root@test:/tmp# iasl -d spcr.dat
Intel ACPI Component Architecture
ASL+ Optimizing Compiler version 20160831-64
Copyright (c) 2000 - 2016 Intel Corporation
Input file spcr.dat, Length 0x50 (80) bytes
ACPI: SPCR 0x0000000000000000 000050 (v01 BHYVE BVSPCR 00000001 BHYV 00000001)
Acpi Data Table [SPCR] decoded
Formatted output: spcr.dsl - 2599 bytes
root@test:/tmp# cat spcr.dsl
/*
* Intel ACPI Component Architecture
* AML/ASL+ Disassembler version 20160831-64
* Copyright (c) 2000 - 2016 Intel Corporation
*
* Disassembly of spcr.dat, Sun Mar 11 17:44:37 2018
*
* ACPI Data Table [SPCR]
*
* Format: [HexOffset DecimalOffset ByteLength] FieldName : FieldValue
*/
[000h 0000 4] Signature : "SPCR" [Serial Port Console Redirection table]
[004h 0004 4] Table Length : 00000050
[008h 0008 1] Revision : 01
[009h 0009 1] Checksum : 78
[00Ah 0010 6] Oem ID : "BHYVE "
[010h 0016 8] Oem Table ID : "BVSPCR "
[018h 0024 4] Oem Revision : 00000001
[01Ch 0028 4] Asl Compiler ID : "BHYV"
[020h 0032 4] Asl Compiler Revision : 00000001
[024h 0036 1] Interface Type : 00
[025h 0037 3] Reserved : 000000
[028h 0040 12] Serial Port Register : [Generic Address Structure]
[028h 0040 1] Space ID : 01 [SystemIO]
[029h 0041 1] Bit Width : 08
[02Ah 0042 1] Bit Offset : 00
[02Bh 0043 1] Encoded Access Width : 00 [Undefined/Legacy]
[02Ch 0044 8] Address : 00000000000003F8
[034h 0052 1] Interrupt Type : 01
[035h 0053 1] PCAT-compatible IRQ : 04
[036h 0054 4] Interrupt : 00000000
[03Ah 0058 1] Baud Rate : 07
[03Bh 0059 1] Parity : 00
[03Ch 0060 1] Stop Bits : 01
[03Dh 0061 1] Flow Control : 03
[03Eh 0062 1] Terminal Type : 02
Notice that it seems to have the address for COM1 (0x3F8), IRQ 4, etc., which
all seems legit for my setup. If all of this available when a machine redirects
a serial console, why have I been manually configuring serial consoles all these
years?
Let me predict the most common reasons:
- It is somethign “new” that Microsoft came up with. By “new” I mean sometime in the past decade.
- The BIOS implementations that support it are so buggy that the information is not reliable
FWIW, the specification is available in .docx format from Microsoft.
Written on March 11, 2018
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