Re: [Qemu-devel] [kvmtool test PATCH 22/24] kvmtool: arm64: Add support for guest physical address size

[Auger Eric]

Hi Marc,

On 07/05/2018 03:20 PM, Marc Zyngier wrote:
> On 05/07/18 13:47, Julien Grall wrote:
>> Hi Will,
>>
>> On 04/07/18 16:52, Will Deacon wrote:
>>> On Wed, Jul 04, 2018 at 04:00:11PM +0100, Julien Grall wrote:
>>>> On 04/07/18 15:09, Will Deacon wrote:
>>>>> On Fri, Jun 29, 2018 at 12:15:42PM +0100, Suzuki K Poulose wrote:
>>>>>> Add an option to specify the physical address size used by this
>>>>>> VM.
>>>>>>
>>>>>> Signed-off-by: Suzuki K Poulose <address@hidden>
>>>>>> ---
>>>>>>   arm/aarch64/include/kvm/kvm-config-arch.h | 5 ++++-
>>>>>>   arm/include/arm-common/kvm-config-arch.h  | 1 +
>>>>>>   2 files changed, 5 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/arm/aarch64/include/kvm/kvm-config-arch.h 
>>>>>> b/arm/aarch64/include/kvm/kvm-config-arch.h
>>>>>> index 04be43d..dabd22c 100644
>>>>>> --- a/arm/aarch64/include/kvm/kvm-config-arch.h
>>>>>> +++ b/arm/aarch64/include/kvm/kvm-config-arch.h
>>>>>> @@ -8,7 +8,10 @@
>>>>>>                          "Create PMUv3 device"),                         
>>>>>> \
>>>>>>          OPT_U64('\0', "kaslr-seed", &(cfg)->kaslr_seed,                 
>>>>>> \
>>>>>>                          "Specify random seed for Kernel Address Space " 
>>>>>> \
>>>>>> -                        "Layout Randomization (KASLR)"),
>>>>>> +                        "Layout Randomization (KASLR)"),                
>>>>>> \
>>>>>> +        OPT_INTEGER('\0', "phys-shift", &(cfg)->phys_shift,             
>>>>>> \
>>>>>> +                        "Specify maximum physical address size (not "   
>>>>>> \
>>>>>> +                        "the amount of memory)"),
>>>>>
>>>>> Given that this is a shift value, I think the help message could be more
>>>>> informative. Something like:
>>>>>
>>>>>   "Specify maximum number of bits in a guest physical address"
>>>>>
>>>>> I think I'd actually leave out any mention of memory, because this does
>>>>> actually have an effect on the amount of addressable memory in a way that 
>>>>> I
>>>>> don't think we want to describe in half of a usage message line :)
>>>> Is there any particular reasons to expose this option to the user?
>>>>
>>>> I have recently sent a series to allow the user to specify the position
>>>> of the RAM [1]. With that series in mind, I think the user would not really
>>>> need to specify the maximum physical shift. Instead we could automatically
>>>> find it.
>>>
>>> Marc makes a good point that it doesn't help for MMIO regions, so I'm trying
>>> to understand whether we can do something differently there and avoid
>>> sacrificing the type parameter.
>>
>> I am not sure to understand this. kvmtools knows the memory layout 
>> (including MMIOs) of the guest, so couldn't it guess the maximum 
>> physical shift for that?
> 
> That's exactly what Will was trying to avoid, by having KVM to compute
> the size of the IPA space based on the registered memslots. We've now
> established that it doesn't work, so what we need to define is:
> 
> - whether we need another ioctl(), or do we carry on piggy-backing on
> the CPU type,
kvm type I guess
> - assuming the latter, whether we can reduce the number of bits used in
> the ioctl parameter by subtly encoding the IPA size.
Getting benefit from your Freudian slip, how should guest CPU PARange
and maximum number of bits in a guest physical address relate?

My understanding is they are not correlated at the moment and our guest
PARange is fixed at the moment. But shouldn't they?

On Intel there is
   qemu-system-x86_64 -M pc,accel=kvm -cpu SandyBridge,phys-bits=36
or
   qemu-system-x86_64 -M pc,accel=kvm -cpu SandyBridge,host-phys-bits=true

where phys-bits, as far as I understand has a a similar semantics as the
PARange.

Thanks

Eric
> 
> Thanks,
> 
>       M.
>