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Re: [Xen-devel] Questions about PVHv2/HVMlite

On Tue, May 16, 2017 at 6:13 PM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
> On 05/16/2017 11:52 AM, Gary R Hook wrote:
>> On 05/15/2017 09:54 PM, Boris Ostrovsky wrote:
>> Possibly stupid question time...
>>> On 05/15/2017 03:51 PM, Gary R Hook wrote:
>>>>   2) Or, perhaps more importantly, what distinguishes said guest?
>>> Simplifying things a bit, it's an HVM guest that doesn't have device
>>> model (i.e. qemu) and which is booted directly (i.e. without hvmloader)
>> So, an unmodified/stock kernel which would rely upon a typical (i.e. its
>> own grub) bootloader. The magic comes from the PVH drivers?
> Typically there is no bootloader (or, one might say, the hypervisor is
> the bootloader). You indicate which kernel to boot in the config file
> (just like for PV guests).
> I believe there is some work going on with OVMF that will make it boot
> in PVH mode. It will then mount the guest filesystem and load the
> kernel. I think that's what Andrew was referring to.

All this is quite confusing to a typical user because from Xen's
perspective, PV and HVM and PVH aren't as different as they look from
a user's perspective.

The most common mode of operation for HVM guests is to run SeaBIOS ->
grub -> kernel inside the guest.  But you *can* direct-boot an HVM

And the most common mode of operation for PV is to direct boot it
(usually by running pygrub in dom0).  But you *can* also run grub
inside the PV guest -- but only because grub has been ported to run in
PV mode*.

In both cases, from the hypervisor's perspective, the only difference
is which blob of data is written into the guest when it starts: a
kernel or something which loads a kernel.

The same will be true for PVH: You'll be able to direct-boot it
(either by specifying a kernel or pygrub).  We're also working on
porting OVMF to PVH mode, so that nothing in dom0 has to interpret the
guest's filesystem.  In theory we could port grub or even SeaBIOS to
run in PVH mode as well.

At the moment this is controlled by a mish-mash of different
configuration parameters and you have to know far too much about the
internals.  Once the 4.10 development window opens up we'll be trying
to make a more sensible interface.


* But you have to know ahead of time whether the kernel you will
eventually boot will be 64-bit or 32-bit, because PV guests can't
change mode.

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