[Bug 215740] [bhyve] utilizing passthru breaks raw device usage with virtio-blk | ahci-hd

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215740

            Bug ID: 215740
           Summary: [bhyve] utilizing passthru breaks raw device usage
                    with virtio-blk | ahci-hd
           Product: Base System
           Version: 11.0-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: misc
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: bugzilla.freebsd at omnilan.de
                CC: freebsd-amd64 at FreeBSD.org
                CC: freebsd-amd64 at FreeBSD.org

Using a passthru device with bhyve(8) for hosting guests with a physical device
as storage backend (regardless if accessed through virtio-blk or ahci-hd)
corrupts guest-disk access, while file-backed ahci-hd (or virtio-blk) doesn't
show that problem with passthru.

Steps to reproduce:

Use any harddrive containing any installed OS.
On the host: 'hd /dev/ada6 | less'
See MBR/PMBR code.

Use the same device (ada6 in that example) and conncet it to a FreeBSD-Live-DVD
guest with a passthru device involved
(e. g.
bhyveload -d ./releases/ISO-IMAGES/11.0/FreeBSD-11.0-RELEASE-amd64-disc1.iso -S
-m 2G ppttest && bhyve -u -A -H -P -s 0,hostbridge -s
3,ahci,cd:./releases/ISO-IMAGES/11.0/FreeBSD-11.0-RELEASE-amd64-disc1.iso,hd:/dev/ada6
-s 5,passthru,0/25/0 -s 31,lpc -l com1,stdio -S -m 2G -c 4 ppttest
)

Inside the guest, 'hd /dev/ada0 | less' doesn't work anymore (endless I/O)
Using 'dd if=/dev/ada6 count=1 | hd' shows only 0x0 instead of the output you
saw on the host!

Simply repeating this without the passthru device in place solves the problem,
you see exactly the same bytes inside the guest as on the host.

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