[Xen-devel] RE: Live migration fails due to c/s 20627

["Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>]

HVM RDTSCP fix.
 - Put the guest rdtscp cpuid logic in xc_cpuid_x86.c.
 - MSR_TSC_AUX's high 32bit is reserved, so only write the low 32bit.

Signed-off-by: Dongxiao Xu <dongxiao.xu@xxxxxxxxx>



Xu, Dongxiao wrote:
> Keir Fraser wrote:
>> On 14/12/2009 18:02, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>
>> wrote: 
>> 
>>> This may be true in concept, but existing tools (including
>>> the default xm tools) do NOT check for this... I just
>>> tested a live migration between a Nehalem (which supports
>>> rdtscp) and a Conroe (which does not).  The live migration
>>> works fine and the app using rdtscp runs fine on the
>>> Nehalem and then crashes when the live migration completes
>>> on the Conroe.  I *know* of existing code in Oracle
>>> that will be broken by this!
>> 
>> This is a general problem for migration between dissimilar
>> processors. The solution is to 'level' the feature sets, by masking
>> CPUID flags from the more-featured processor. In this case you would
>> mask out RDTSCP (and perhaps others too). This does need the RDTSCP
>> flag setting/clearing to be moved to xc_cpuid_x86.c, as currently the
>> user cannot override the policy wedged into the hypervisor itself.
>> That's an easy thing to fix.
> 
> I will write a patch for this. Thanks!
> 
> -- Dongxiao
> 
>> 
>>  -- Keir
>> 
>> 
>> 
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Attachment: hvm_rdtscp.patch
Description: hvm_rdtscp.patch

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