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

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Date:	Fri, 26 Feb 2010 14:04:01 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Alexander Graf <agraf@...e.de>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] KVM: SVM: Optimize nested svm msrpm merging

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.

	Joerg


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