Re: [PATCH] kvm-s390: userspace access to guest storage keys

[Christian Borntraeger <cborntra@de.ibm.com>]

On 09/05/11 13:32, Alexander Graf wrote:
> 
> On 09.05.2011, at 13:20, Avi Kivity wrote:
> 
>> On 05/09/2011 01:11 PM, Alexander Graf wrote:
>>>>
>>>>  And not in main memory, either?
>>>
>>> Nope - storage keys are only accessible using special instructions. They're not in RAM (visible to a guest) :).
>>>
>>
>> Interesting, so where are they kept?  An on-chip memory?  That would limit the amount of main
>> memory to that indexed by the chip.  Extra off-chip memory?
> 
> I'll leave the answer of this question to the real experts :). All I know is that the ISA defines special instructions
> to fetch them, so the question on where they are stored is "implementation dependent". In qemu for example, they are 
> kept in an extra array that's just malloc'ed.

Dont know really - the HW folks probably know it better, so I am just guessing.
The final location is probably somewhere in the HSA (hardware system area) which is backed by
real memory and hidden by the cpu microcode.  (there is no way to access this memory directly).
But parts of the storage keys are also mirrored by the different cache levels and TLB (e.g. write
protection via page tables).


Christian