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Re: [Qemu-devel] [PATCH] resent: x86/cpuid: propagate further CPUID leaf

From: Andre Przywara
Subject: Re: [Qemu-devel] [PATCH] resent: x86/cpuid: propagate further CPUID leafs when -cpu host
Date: Tue, 25 May 2010 15:21:46 +0200
User-agent: Thunderbird (X11/20090820)

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<address@hidden>

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.


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

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