Re: [Xen-devel] Re: Xen: Hybrid extension patchset for hypervisor

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Wed, 2009-09-16 at 22:22 +0100, Jeremy Fitzhardinge wrote:
> On 09/16/09 14:12, Ian Campbell wrote:
> >>> Based on our data, what we would want in PV 64-bit guests are, 
> >>> fundamentally:
> >>> - have the kernel run in ring 0 (so that it can regain the performance 
> >>> enhancements)
> >>>   
> >>>       
> >> That's no problem.  PV kernels don't currently assume they're running in
> >> any particular ring, so they'd be happy to run in ring 0 if that's how
> >> they're started (if there are problems, I'd consider that a bug).  We
> >> could then check for ring 0 and enable syscall/sysenter.
> >>     
> > XENFEAT_supervisor_mode_kernel is supposed to enable this behaviour,
> > although it hasn't been actively used for several years and never in the
> > pvops kernel so you can bet it has bit-rotted...
> >   
> 
> That tends to have a slightly different meaning, viz "dom0 really *is*
> privileged and can do anything it feels like".  It isn't necessary to
> have a specific feature/mechanism for "kernel happens to be in ring 0";
> it can look at its own cs ring number.

In practise, at least for the 2.6.18-xen tree (which is the only one
where I expect it was ever completely implemented), it is only used to
set the kernel CS and DS and to gate sysenter setup (for which I think
we have a better mechanism today) but you are right that in principle it
could be more far reaching than that.

> >> We could do that with minimal API/ABI changes by:
> >>
> >>     * Providing an identity p2m table
> >>     * Changing the hypercall page to make pte writes simple memory
> >>       writes (no hypercalls); xen would still keep track of pinned pages
> >>       and trap'n'emulate on them for back-compatibility (but fast-path
> >>       with no validation).  We could expose the presence of HAP via
> >>       xen_features so that guests know they can avoid marking pagetables
> >>       RO, etc.
> >>     * Similarly, cr3 changes can be fast-pathed within the hypercall page.
> >>     * Whatever else I've overlooked.
> >>     
> > Some combination of XENFEAT_writable_page_tables
> > XENFEAT_writable_descriptor_tables and XENFEAT_auto_translated_physmap
> > might be of interest for this bit.
> 
> Making use of XENFEAT_auto_translated_physmap would avoid the need for
> identity p2m/m2p tables, but I'm not sure whether it still works.  I got
> close to completely removing all references to it at one point, but I
> think ia64 uses it?

I very much expect that it'll need fixing/(re)implementing on both the
kernel and hypervisor side...

Ian.


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