[Xen-users] Xen unable to attach root SCSI disk (macbook air)

[Tim Cuthbertson <tim@xxxxxxxxxxx>]

Hi all,

I'm trying to get a Xen install with NixOS as dom0 on a 2013 macbook
air. I've come at it somewhat gently - I've got xen+nixos working in
virtualbox, then progressed to vanilla nixos on the baremetal (dual
booting alongside OSX). Now I'm trying to boot xen+nixos on the
baremetal, and it's not working. Specifically, it's unable to mount my
root disk. And it seems to be a very low-level failure - /sys/block is
completely empty and /dev/sd* never gets populated.

I've done some digging in dmesg, and it seems like the most relevant section is:

---

SCSI subsystem initialized
libata version 3.00 loaded.
ahci 0000:04:00.0: version 3.0
xen: registering gsi 16 triggering polarity 1
Already setup the GSI :16
ahci 0000:04:00.0: AHCI 0001.0000 32 slots 1 ports 6 Gbps 0x1 impl SATA mode
ahci 0000:04:00.0: flags: 64bit ncq sntf led pio slum part
scsi host0: ahci
ata1: SATA max UDMA/133 abar m512@0xb0700000 port 0xb0700100 irq 47
ata1: SATA link up 6.0 Gbps (SStatus 133 SControl 300)
ata1.00: failed to IDENTIFY (INIT_DEV_PARAMS failed, err_mask=0x80)

----

Since I have a working nixos install (which in theory only differs by
the presence of xen), here's the corresponding dmesg snippet matching
the above, from a working non-xen boot:

----

SCSI subsystem initialized
libata version 3.00 loaded.
ahci 0000:04:00.0: version 3.0
ahci 0000:04:00.0: irq 47 for MSI/MSI-X
ahci 0000:04:00.0: AHCI 0001.0000 32 slots 1 ports 6 Gbps 0x1 impl SATA mode
ahci 0000:04:00.0: flags: 64bit ncq sntf led pio slum part
scsi host0: ahci
ata1: SATA max UDMA/133 abar m512@0xb0700000 port 0xb0700100 irq 47
ata1: SATA link up 6.0 Gbps (SStatus 133 SControl 300)
ata1.00: unexpected _GTF length (8)
ata1.00: ATA-8: APPLE SSD SD0128F, A222021, max UDMA/133
ata1.00: 236978176 sectors, multi 1: LBA48 NCQ (depth 31/32), AA
ata1.00: unexpected _GTF length (8)
ata1.00: configured for UDMA/133
scsi 0:0:0:0: Direct-Access     ATA      APPLE SSD SD0128 021  PQ: 0 ANSI: 5
sd 0:0:0:0: [sda] 236978176 512-byte logical blocks: (121 GB/113 GiB)
sd 0:0:0:0: [sda] 4096-byte physical blocks
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't
support DPO or FUA
 sda: sda1 sda2 sda3 sda4 sda5
sd 0:0:0:0: [sda] Attached SCSI disk
usb 1-3: new full-speed USB device number 2 using xhci_hcd
EXT4-fs (sda5): mounted filesystem with ordered data mode. Opts: (null)
EXT4-fs (sda5): re-mounted. Opts: (null)

----

I don't know what differences to expect when booting xen vs non-xen.
But I'd assume that at this early stage of booting, the low level
device drivers should be the same, and so there shouldn't be anything
xen-specific I'd need to do to get this disk recognised. But perhaps
there is something I need to do to have the xen-enabled kernel
recognise this device? Any ideas?

In case a bit more background is useful, I'm booting from EFI with
gummiboot, which is supported in NixOS normally but is _not_ supported
in Xen+NixOS. So I did have to do some changes to get a xen efi image
(i.e. a custom binutils compile, plus creating the EFI .cfg somewhat
manually). But I a getting far enough to get the kernel + initrd all
running, so I think that stuff is all working well enough. My Xen is
version 4.5.1, and a 3.18.20 kernel.

Thanks,
 - Tim.

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