Re: [Xen-users] Questions on hvmloader, direct kernel boot and simulated BIOS

[George Dunlap <dunlapg@xxxxxxxxx>]

On Wed, Sep 14, 2016 at 4:42 PM, Brett Stahlman <brettstahlman@xxxxxxxxx> wrote:
> 2. I understand the need for ovmf for a UEFI-booted guest on a
> BIOS-booted host, but is the seabios firmware blob strictly necessary
> in the case of a BIOS-booted host running a BIOS-booted guest? I mean,
> what would happen if, in lieu of the firmware copy, xen set up a "flat
> mapping" in the guest (i.e., virtual=physical) for the firmware range
> 0xF0000-0x100000? I.e., could it not simply use the actual host
> firmware? Or are there some writable locations in that range that
> would preclude this possibility?

Just to emphasize the point that Austin made: The 'virtual' in
'virtual machine' means "fake".  The virtual machine has virtual
physical memory which has no direct relationship to the host physical
memory; the virtual PCI and platform devices are emulated by Xen and
qemu and have no relationship to the physical PCI and platform devices
on the host system; any virtual devices available to the guest which
are hooked in to that PCI device are emulated by qemu -- virtual disk
controller, virtual network card, virtual USB controller.  (And in
fact, in Xen, we're using quite an old emulated PCI controller.)
SeaBIOS and OVMF are firmware specifically designed to run inside such
a virtual environment.

 -George

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