Re: [Xen-devel] HVMLite / PVHv2 - using x86 EFI boot entry

[Konrad Rzeszutek Wilk]

On Thu, Apr 14, 2016 at 11:12:01PM +0200, Luis R. Rodriguez wrote:
> On Thu, Apr 14, 2016 at 04:38:47PM -0400, Konrad Rzeszutek Wilk wrote:
> > > This has nothing to do with dominance or anything nefarious, I'm asking
> > > simply for a full engineering evaluation of all possibilities, with
> > > the long term in mind. Not for now, but for hardware assumptions which
> > > are sensible 5 years from now.
> > 
> > There are two different things in my mind about this conversation:
> > 
> >  1). semantics of low-level code wrapped around pvops. On baremetal
> >    it is easy - just look at Intel and AMD SDM.
> >    And this is exactly what running in HVM or HVMLite mode will do -
> >    all those low-level operations will have the same exact semantic
> >    as baremetal.
> 
> Today Linux is KVM stupid for early boot code. I've pointed this out

-EPARSE?
> before, but again, there has been no reason found to need this. Perhaps
> for HVMLite we won't need this...

Are you talking about kvmtools? Which BTW are similar to how HVMLite
would expose the platform.
> 
> >    There is no hope for the pv_ops to fix that.
> 
> Actually I beg to differ. See my patches and ongoing work.

I meant in terms of semantics. As in I cannot see some of
those pv-ops to have the same semantics as baremetal. For example
set_pte is simple on x86 (movq $<some value>, <memory address>).

While on Xen PV it is a potential batching hypercall with
lookup in an P2M table, then perhaps a sidelong look at
the M2P, then maybe the M2P override.

> 
> >    And I am pretty sure the HVMLite in 5 years will have no
> >    trouble in this as it will be running in VMX mode (HVM).
> 
> HVMLite may still use PV drivers for some things, its not super
> obvious to me that low level semantics will not be needed yet.

PV drivers are very different from low-level semantics.

And it will have to use them.

Maybe it is easier to think of this in terms of kvmtool - it
is pretty much how this would work - but instead of VirtIO
drivers you would be using the Xen PV drivers (thought one
could also use VirtIO ones if you wanted).
> 
> >  2). Boot entry.
> > 
> >    The semantics on Linux are well known - they are documented in
> >    Documentation/x86/boot.txt.
> > 
> >    HVMLite Linux guests have to somehow provide that.
> > 
> >    And how it is done seems to be tied around:
> > 
> >    a) Use existing boot paths - which means making some
> >       extra stub code to call in those existing boot paths
> >       (for example Xen could bundle with an GRUB2-alike
> >        code to be run when booting Linux using that boot-path).
> > 
> >       Or EFI (for a ton more code). Granted not all OSes
> >       support those, so not very OS agnostic.
> 
> What other OSes do is something to consider but if they don't
> do it because they are slacking in one domain should by no means
> be a reason to not evaluate the long term possible gains.
> Specially if we have reasons to believe more architectures will
> consider it and standardize on it.
> 
> It'd be silly not to take this a bit more seriously.

Complexity vs simplicity.
> 
> >        Hard part - if the bootparams change then have to
> >       rev up the code in there. May be out of sync
> >       with Linux bootparams.
> 
> If we are going to ultimately standardize on EFI boot for new
> hardware it'd be rather silly to extend the boot params further.

Whoa there... Have you spoken to hpa,tglrx about this?

> 
> >    b) Add another simpler boot entry point which has to copy
> >      "some" strings from its format in bootparams.
> > 
> > 
> >    So this part of the discussion does not fall in the
> >    hardware assumptions. Intel SDM or AMD mention nothing about
> >    boot loaders or how to boot an OS - that is all in realms
> >    of how software talks to software.
> 
> Right -- so one question to ask here is what other uses are there
> for this outside of say HVMLite. You mentioned Multiboot so far.
> 
> >  3). And there is the discussion on man-power to make this
> >    happen.
> 
> Sure.
> 
> >  4). Lastly which one is simpler and involves less code so
> >     that there is a less chance of bitrot.
> 
> Indeed.
> 
> You also forgot the tie-in between dead-code and semantics but

Wait, I just spoke about CPU semantics?! Which semantics
are you talking about?
> that clearly is not on your mind. But I'd say this is a good
> summary.

I put 'dead code' in the same realm as device drivers work.
And they seem to always have some issue or another.
Or maybe I getting unlucky and getting copied on those bugs.
> 
>   Luis