Re: [Xen-devel] [PATCH v14 00/20] Introduce PVH domU support

[Tim Deegan <tim@xxxxxxx>]

At 15:53 +0000 on 08 Nov (1383922404), George Dunlap wrote:
> On 08/11/13 15:41, George Dunlap wrote:
> > On 04/11/13 17:34, Tim Deegan wrote:
> >> At 17:23 +0000 on 04 Nov (1383582187), George Dunlap wrote:
> >>> On 04/11/13 16:59, Konrad Rzeszutek Wilk wrote:
> >>>>> This also means no support for "legacy" forced invalid ops -- only 
> >>>>> native
> >>>>> cpuid is supported in this series.
> >>>> OK.
> >>> (FWIW, support for legacy forced invalid ops was requested by Tim.)
> >> I was worried about existing PV kernel code that used the fake-CPUID,
> >> which would break if the 'core' kernel code went from PV to PVH.  But
> >> I guess I could be convinced that such kernel code is buggy? Really,
> >> the high-order bit was consistency.  The version I commented on
> >> supported them for user-space but not for kernel, which seemed like
> >> risking trouble for no benefit.
> >
> > Oh right -- I think Mukesh we do need to support forced invalid ops 
> > for user space so that we can use the same xen tools binaries on PV 
> > and PVH kernels.
> >
> > Hmm, Mukesh / Konrad, what tools are you actually thinking about 
> > here?  Are these Oracle-specific tools?  I can't seem to find 
> > XEN_CPUID or XEN_EMULATE_PREFIX anywhere in the tools/ directory of 
> > the xen repo...
> 
> It's misc/xen-detect.c, for the curious (which helpfully does not use 
> the macros above).
> 
> Handling forced invalid ops it is, then.

The xen-detect code tries real CPUID first, so not supporting the fake
CPUID doesn't actually affect it at all.  That's why I could be
convinced that detection failures were a guest-side bug.

It would affect older code that _only_ tired the fake CPUID, or code
that for some reason needed to behave differently on PV vs HVM (though
it's not at all clear that on PVH such code should use the 'PV'
behaviour.

Tim.


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