Re: [Xen-devel] [QUERY] lguest64

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

> >>>          struct pv_cpu_ops pv_cpu_ops;                                    
> >>>        
> >>>           [only end up using cpuid. This one is a tricky one. We could
> >>>            arguable remove it but it does do some filtering - for example
> >>>            THERM is turned off, or MWAIT if a certain hypercall tells us 
> >>> to
> >>>            disable that. Since this is now a trapped operation this could 
> >>> be
> >>>            handled in the hypervisor - but then it would be in charge of
> >>>            filtering certain CPUID - and this is at bootup - so there is 
> >>> not
> >>>            user interaction. This needs a bit more of thinking]
> >>>
> >> read_msr/write_msr in this one make all msr accesses safe. IIRC there
> >> are MSRs that Linux uses without checking cpuid bits.
> >> IA32_PERF_CAPABILITIES for instance is used without checking PDCM bit.
> > 
> > Right, those are needed as well. Completly forgot about them.
> 
> CPUID is not too bad.  RDMSR/WRMSR is actually worse since there are
> some MSRs which are performance-critical.  The really messy pvops are
> the memory-related ones, as they don't match the hardware behavior.

Would you have a by any chance a nice test-case to demonstrate the
rdmsr/wrmsr paths which performance-critical under baremetal?
> 
> Similarly, beyond pvops, what new assumptions does this code add to the
> code base?

We have not yet narrowed down on how to "negotiate" the GDT values - as
the VMX code in the hypervisor has setup those before it loads the kernel.
I think Mukesh was thinking to extend the .Xen.note to enumerate some of the
ones that are needed and somehow the hypervisor slurps them in.


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