[Xen-devel] Re: [PATCH] cpuid handling for guest

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 9 Jun 2006, at 21:12, Kamble, Nitin A wrote:

> Hi Keir, Ian,
>    Attached is a patch to provide right view of cpuid to the HVM 
> guests. Some of the CPU features such as APIC, PAE, MTRR, HT are 
> virtualized; while others are not virtualized yet such as TM1, TM2, 
> MCA and there are some features which do not need virtualization such 
> as MMX. With the patch Guest sees only those processor features in the 
> cpuid which are virtualized in the hyper visor, or do not need any 
> virtualization in hyper visor.
Accepted because it's fine as far as it goes, but this emulation of 
CPUID is unsafe because you do not restrict the 'max leaf' values (as 
returned in %eax by 0x0 and 0x80000000). So current HVM support cannot 
be guaranteed to work on future processors because they may have extra 
info leaves that you do not filter and emulate properly (because the 
code cannot know about them).
At the very least you should be restricting max leaves to 0xA and 
0x80000008 (highest documented leaves so far). You should also emulate 
native behaviour of returning info of max basic leaf for invalid CPUID 
indexes.
But, while doing this you may as well make your lives easier and only 
advertise max leaves 0x3 and 0x80000004. This will avoid hassle of 
properly emulating other leaves, many of which contain performance info 
that may not be valid across save/restore and migration anyway.
 -- Keir


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