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

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Date:	Fri, 26 Feb 2010 13:28:29 +0100
From:	Alexander Graf <agraf@...e.de>
To:	Joerg Roedel <joerg.roedel@....com>
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 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.

Alex
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