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

[Brett Stahlman <brettstahlman@xxxxxxxxx>]

On Mon, Sep 19, 2016 at 3:36 AM, George Dunlap <dunlapg@xxxxxxxxx> wrote:
> 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.

Hmm... I did understand the distinction between virtual and host
physical memory; what I was missing was the special nature of
SeaBIOS/OVMF firmware: in particular, I was under the mistaken
impression that SeaBIOS could run directly on hardware without
something like QEMU or Coreboot. I appreciate the clarification...

Thanks,
Brett S.

>
>  -George

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