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Date: Fri, 26 Feb 2010 14:42:36 +0200
From: Avi Kivity <avi@redhat.com>
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To: Joerg Roedel <joerg.roedel@amd.com>
CC: Marcelo Tosatti <mtosatti@redhat.com>, Alexander Graf <agraf@suse.de>,
       kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/5] KVM: SVM: Optimize nested svm msrpm merging
References: <1267118149-15737-1-git-send-email-joerg.roedel@amd.com> <1267118149-15737-3-git-send-email-joerg.roedel@amd.com> <4B87A248.1050300@redhat.com> <20100226122502.GC12689@amd.com>
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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.