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

[Joerg Roedel]

On Fri, Feb 26, 2010 at 02:08:25PM +0100, Alexander Graf wrote:
> 
> On 26.02.2010, at 14:04, Joerg Roedel wrote:
> 
> > On Fri, Feb 26, 2010 at 01:28:29PM +0100, Alexander Graf wrote:
> >> 
> >> On 26.02.2010, at 13:25, 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.
> >> 
> >> I'm still not convinced on this way of doing things. If it's static,
> >> make it static. If it's dynamic, make it dynamic. Dynamically
> >> generating a static list just sounds plain wrong to me.
> > 
> > Stop. I had a static list in the first version of the patch. This list
> > was fine except the fact that a developer needs to remember to update
> > this list if the list of non-intercepted msrs is expanded. The whole
> > reason for a dynamically built list is to take the task of maintaining
> > the list away from the developer and remove a possible source of hard to
> > find bugs. This is what the current approach does.
> 
> I was more thinking of replacing the function calls with a list of
> MSRs. You can then take that list on module init, generate the MSR
> bitmap once and be good.

The msr-bitmap is per-vcpu tu support lbr-virtualization. The access to
the lbr-msrs is only enabled if the guest-vcpu enabled lbr-debugging.

A list of MSRs keeps the problem that the information is maintained at
two places: the list and the various set_msr_intercept() function calls.

	Joerg


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