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