Re: [Qemu-devel] [PATCH] resent: x86/cpuid: propagate further CPUID leafs when -cpu host

[Andre Przywara <andre.przywara@amd.com>]

Anthony Liguori wrote:
> On 05/21/2010 02:50 AM, Andre Przywara wrote:
>> -cpu host currently only propagates the CPU's family/model/stepping,
>> the brand name and the feature bits.
>> Add a whitelist of safe CPUID leafs to let the guest see the actual
>> CPU's cache details and other things.
>>
>> Signed-off-by: Andre Przywara<andre.przywara@amd.com>
>>    
> 
> The problem I can see is that this greatly increases the chances of 
> problems with live migration since we don't migrate the cpuid state.
I think that should be fixed. Although -cpu host is not a wise choice 
for migration, even without these additional leaves the feature bits 
probably don't match between source and target.

> What's the benefit of exposing this information to the guest?

That is mostly to propagate the cache size and organization parameters 
to the guest:
 >> +/* safe CPUID leafs to propagate to guest if -cpu host is specified
 >> + * Intel defined leafs:
 >> + * Cache descriptors (0x02)
 >> + * Deterministic cache parameters (0x04)
 >> + * Monitor/MWAIT parameters (0x05)
 >> + *
 >> + * AMD defined leafs:
 >> + * L1 Cache and TLB (0x05)
 >> + * L2+L3 TLB (0x06)
 >> + * LongMode address size (0x08)
 >> + * 1GB page TLB (0x19)
 >> + * Performance optimization (0x1A)
 >> + */
Since at least L1 and L2 caches are mostly private to vCPUs, I see no 
reason to disguise them.

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12