Re: [PATCH 2/5] KVM: SVM: Optimize nested svm msrpm merging

[Avi Kivity]

On 02/26/2010 02:25 PM, Joerg Roedel wrote:
> On Fri, Feb 26, 2010 at 12:28:24PM +0200, Avi Kivity wrote:
>    
> > > +static void add_msr_offset(u32 offset)
> > > +{
> > > +	u32 old;
> > > +	int i;
> > > +
> > > +again:
> > > +	for (i = 0; i<   MSRPM_OFFSETS; ++i) {
> > > +		old = msrpm_offsets[i];
> > > +
> > > +		if (old == offset)
> > > +			return;
> > > +
> > > +		if (old != MSR_INVALID)
> > > +			continue;
> > > +
> > > +		if (cmpxchg(&msrpm_offsets[i], old, offset) != old)
> > > +			goto again;
> > > +
> > > +		return;
> > > +	}
> > > +
> > > +	/*
> > > +	 * If this BUG triggers the msrpm_offsets table has an overflow. Just
> > > +	 * increase MSRPM_OFFSETS in this case.
> > > +	 */
> > > +	BUG();
> > > +}
> > >        
> > Why all this atomic cleverness?  The possible offsets are all
> > determined statically.  Even if you do them dynamically (makes sense
> > when considering pmu passthrough), it's per-vcpu and therefore
> > single threaded (just move msrpm_offsets into vcpu context).
> >      
> The msr_offset table is the same for all guests. It doesn't make sense
> to keep it per vcpu because it will currently look the same for all
> vcpus. For standard guests this array contains 3 entrys. It is marked
> with __read_mostly for the same reason.
>    

In that case, you can calculate it during module initialization.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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