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Re: [libvirt] [RFC] powerpc : Add support for VM in compat mode



On Thursday 26 June 2014 03:41 PM, Prerna Saxena wrote:
> PowerISA allows processors to run VMs in binary compatibility ("compat")
> mode supporting an older version of ISA. 
> Eg,in compatibility mode,  a POWER8 host can run a "Power7" VM,conforming
> to PowerISA v2.06, while a POWER7 host can run a "POWER6" VM, conforming
> to PowerISA v2.05.
>
> QEMU has recently added support to explicitly denote a VM running in
> compatibility mode through commit 6d9412ea. Now, a "compat" mode VM can
> be run by invoking this qemu commandline on a POWER8 host:
>  -cpu host,compat=power7
> as against the older specification of "-cpu power7".
>
> However, running in compatibility mode is not identical to running
> natively on an older processor. Hence the virtualization stack needs to
> explicitly provide for a compat-mode VM.
>
> This patch allows libvirt to extend the "fallback" semantics of cpu model to
> describe this new mode for PowerKVM guests.
> Additionally with the new scheme of things, qemu doesn't provide a way to query
> for the supported compat models and models returned when querying with '-cpu ?'
> are no longer valid. Hence removing the check cpuModelIsAllowed() for now.
>
> When a user wants to request a power7 vm to run in compatibility mode on
> a Power8 host, this can be described in XML as follows :
>   <cpu mode='custom' match='exact'>
>     <model fallback='compat'>power7</model> 
>   </cpu>
>
> An alternative approach could be to leave the libvirt XML intact, and merely
> change the backend qemu command generation when the VM-requested cpu 
> does not match the host processor.
>
> Looking forward to suggestions on how this can best be implemented..
>
>     Signed-off-by: Li Zhang <zhlcindy linux vnet ibm com>
>     Signed-off-by: Pradipta Kr. Banerjee <bpradip in ibm com>
>     Signed-off-by: Prerna Saxena <prerna linux vnet ibm com>
>
Ping !
Would folks have some feedback on the patch ?

Regards,

-- 
Prerna Saxena

Linux Technology Centre,
IBM Systems and Technology Lab,
Bangalore, India


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