Alok Kataria wrote:

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> Hi Gerd,

>

>> Shouldn't you check the hypervisor signature here?

>

> Nope the whole idea of not checking the hypervisor signature is that we

> should keep this interface generic. 

Nice idea.  Problem with that is that approach is that we don't have

full control here.  It probably isn't that a hard to have vmware, xen

and kvm agree here, given vmware proposes this and for xen+kvm one can

send patches.  But even that you can't take for granted, see the

discussion of the "tsc-may-change-on-migration" problem.
The real big problem are other closed-source hypervisors (VirtualPC /

Hyper-V / Parallels / ...).  How can we be sure they don't define that

leaf to something different?
> Also one thing to remember is, that a hypervisor can decide to not

quoted text
> implement this level and just return "0" the kernel can then just ignore

> that value. That's what we do currently in native_calibrate_tsc.

The fudamental issue outlined above aside:  Even the "ignore 0" part

isn't in the patch right now.
>> Right now both kvm and xen use the first one or two leafes (after info),

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>> but in incompatible ways, so for these the signature *must* be checked

>> before using the info found there.

>

> Hmm that's unfortunate, but we can have exceptions for these one of

> cases and AFAIK these are only checked in the kvm/xen code path and not

> in any generic code as of now, right ? 

Yes.
> btw, i could only find the semantics for 0x40000001 leaf in KVM's header

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> file but don't see Xen using that leaf, can you please point me which

> leafs are you referring to here. 

pv drivers in hvm guests use that (and query very xen-specific stuff

which wouldn't make much sense in other hypervisors).  It isn't in the

kernel source tree, look here instead:
http://xenbits.xensource.com/xen-3.3-testing.hg?file/19201eebab16/unmodi...
cheers,

  Gerd
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