Re: [Xen-devel] [PATCH v4 14/14] xen/x86: setup PVHv2 Dom0 ACPI tables

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Dec 23, 2016 at 10:27:46AM +0000, Roger Pau Monne wrote:
> On Thu, Dec 22, 2016 at 01:19:36PM -0500, Boris Ostrovsky wrote:
> > On 12/22/2016 11:44 AM, Roger Pau Monne wrote:
> > > On Thu, Dec 22, 2016 at 09:24:02AM -0700, Jan Beulich wrote:
> > >>>>> On 22.12.16 at 17:17, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > >>> On 12/22/2016 07:17 AM, Roger Pau Monne wrote:
> > >>>> Maybe Boris has some ideas about how to do CPU hotplug for Dom0?
> > >>> Why would Xen need to be able to parse the AML that is intended to be
> > >>> executed by dom0? I'd think that all the hypervisor would need to do is
> > >>> to load it into memory, not any different from how it's done for regular
> > >>> guests.
> > >> Well, if Dom0 executed the unmodified _MAT, it would get back
> > >> data relating to the physical CPU. Roger is overriding MADT to
> > >> present virtual CPU information to Dom0, and this would likewise
> > >> need to happen for the _MAT return value.
> > 
> > By "unmodified _MAT" you mean system's _MAT?  Why can't we provide our
> > own that will return _MAT object properly adjusted for dom0? We are
> > going to provide our own (i.e. not system's) DSDT, aren't we?
> 
> Providing Dom0 with a different DSDT is almost impossible from a Xen PoV, for
> once Xen cannot parse the original DSDT (because it's a dynamic table), and
> then if we would be to provide a modified DSDT, we would also need an asl
> assembler, so that we could parse the DSDT, modify it, and then compile it
> again in order to provide it to Dom0.
> 
> Although all this code could be put under an init section that would be freed
> after Dom0 creation it seems overkill and very far from trivial, not to 
> mention
> that I'm not even sure what side-effects there would be if Xen parsed the DSDT
> itself without having any drivers.
> 
> > > This is one of the problems with this Dom/Xen0 split brain problem that we
> > > have, and cannot get away from.
> > >
> > > To clarify a little bit, I'm not overwriting the original MADT in memory, 
> > > so
> > > Dom0 should still be able to access it if the implementation of _MAT 
> > > returns
> > > data from that area. AFAICT when plugging in a physical CPU (pCPU) into 
> > > the
> > > hardware, Dom0 should see "correct" data returned by the _MAT method. 
> > > However
> > > (as represented by the " I've used), this data will not match Dom0 vCPU
> > > topology, and should not be used by Dom0 (only reported to Xen in order to
> > > bring up the new pCPU).
> > 
> > So the problem seems to be that we need to run both _MATs --- system's
> > and dom0's.
> 
> Exactly, we need _MAT for pCPUs and _MAT for _vCPUs.
> 
> > > Then the problem arises because we have no way to perform vCPU hotplug for
> > > Dom0, not at least as it is done for DomU (using ACPI), so we would have 
> > > to use
> > > an out-of-band method in order to do vCPU hotplug for Dom0, which is a 
> > > PITA.
> > 
> > 
> > I would very much like to avoid this.
> 
> Maybe we can provide an extra SSDT for Dom0 that basically overwrites the CPU
> objects (_PR.CPUX), but I'm not sure if ACPI allows this kind of objects
> overwrites?
> 
> After reading the spec, I came across the following note in the SSDT section:
> 
> "Additional tables can only add data; they cannot overwrite data from previous
> tables."
> 
> So I guess this is a no-go.
> 
> I only see the following options:
> 
>  - Prevent Dom0 from using the original _MAT methods (or even the full _PR.CPU
>    objects) using the STAO, and then provide Dom0 with an out-of-band method
>    (ie: not ACPI) in order to do CPU hotplug.
> 
>  - Expand the STAO so that it can be used to override ACPI namespace objects,
>    possibly by adding a payload field that contains aml code. It seems that
>    Linux already supports overwriting part of the ACPI namespace from
>    user-space[0], so part of the needed machinery seem to be already in place
>    (hopefully in acpica code?).
> 
>  - Disable the native CPU objects in the DSDT/SSDT using the STAO. Then pick 
> up
>    unused ACPI CPU IDs and use those for vCPUs. Provide an additional SSDT 
> that

You need those so that the C and P state parser can actually parse the other
CPU states which it will upload to Xen.

>    contains ACPI objects for those vCPUs (as is done for DomU). This means we
>    would probably have to start using x2APIC entries in the MADT, since the
>    CPUs IDs might easily expand past 255 (AFAICT we could still keep the APIC
>    IDs low however, since those two are disjoint).
>

- Import ACPI AML parser in Xen. <mad laughter>

 
> I don't really fancy any of these two options, probably the last one seems 
> like
> the best IMHO, but I would like to hear some feedback about them, and of 
> course
> I'm open to suggestions :).

- Do the same thing we had been doing with Xen and Linux PV. Expose the full
  machine MADT and then massage it. Yuck.

I am full of bad ideas today.

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