FreeBSD 7.0 booting with gmirror does not work using SiI SATA controllers

[cjc2007a at xemaps.com]

I posted this on bsdforum.org, but got no response, so here's a repost
(apologies if you have already seen this).  This all started because I was
unable to get gmirror to mirror the boot disks on a SiI3132 PCIe SATA
controller, using FreeBSD 7.0-BETA1.  This controller is supported in 7.0,
and works fine on a single drive.  I have since switched to a JMicron363
PCIe SATA controller.

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I have done some more testing with booting a gmirror RAID1 configuration on
some different disk controllers, both SATA and PATA, and using the PCI and
PCIe busses. I was using FreeBSD 7.0-BETA1.  I followed the instructions in
the FreeBSD handbook.  In summary I installed FreeBSD 7.0 on one drive,
loaded gmirror, created the mirror, and "gmirror status" showed the mirror
to be normal .  I then changed fstab and the /boot/loader.conf files so that
I could boot from the gmirror.  Some controllers would boot the mirror, some
would not.  Here's my results (SiI = Silicon Image):

Success
nVidia nForce 6150 PCIe chipset SATA controller (motherboard)
Command SiI0680 PCI - PATA controller
Promise TX2 PCI - PATA controller
Maxtor (Promise) PCI - PATA controller
JMicron363 - SATA controller

Failure
(Note: geom_mirror.ko loads, but will not find mirror, boot process stalls
at mountroot> prompt)
SiI3112/3114 PCI - SATA controller
SiI3132 PCIe - SATA controller (tried two cards from different vendors)

gmirror works fine using all the tested controllers, but will not survive a
reboot on the two listed above.

I would include a dmesg from one of the failed attempts, but it didn't have
anything that would help. On success the console reads "GEOM_MIRROR: Device
mirror/gm0 launched (1/1). On failure, there is no entry for failing to find
the mirror. The last entry is "trying to mount root from
ufs:/dev/mirror/gm01sa".

So does anyone have any idea why gmirror does not work for a boot disk on
the Silicon Image based SATA controllers? This is baffling.

Greetings,
Chris