Re: [Qemu-devel] [RFC v3 06/15] hw/arm/virt: Allocate device_memory

[David Hildenbrand]

On 05.07.2018 13:42, Auger Eric wrote:
> Hi David,
> 
> On 07/04/2018 02:05 PM, David Hildenbrand wrote:
>> On 03.07.2018 21:27, Auger Eric wrote:
>>> Hi David,
>>> On 07/03/2018 08:25 PM, David Hildenbrand wrote:
>>>> On 03.07.2018 09:19, Eric Auger wrote:
>>>>> We define a new hotpluggable RAM region (aka. device memory).
>>>>> Its base is 2TB GPA. This obviously requires 42b IPA support
>>>>> in KVM/ARM, FW and guest kernel. At the moment the device
>>>>> memory region is max 2TB.
>>>>
>>>> Maybe a stupid question, but why exactly does it have to start at 2TB
>>>> (and not e.g. at 1TB)?
>>> not a stupid question. See tentative answer below.
>>>>
>>>>>
>>>>> This is largely inspired of device memory initialization in
>>>>> pc machine code.
>>>>>
>>>>> Signed-off-by: Eric Auger <address@hidden>
>>>>> Signed-off-by: Kwangwoo Lee <address@hidden>
>>>>> ---
>>>>>  hw/arm/virt.c         | 104 
>>>>> ++++++++++++++++++++++++++++++++++++--------------
>>>>>  include/hw/arm/arm.h  |   2 +
>>>>>  include/hw/arm/virt.h |   1 +
>>>>>  3 files changed, 79 insertions(+), 28 deletions(-)
>>>>>
>>>>> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
>>>>> index 5a4d0bf..6fefb78 100644
>>>>> --- a/hw/arm/virt.c
>>>>> +++ b/hw/arm/virt.c
>>>>> @@ -59,6 +59,7 @@
>>>>>  #include "qapi/visitor.h"
>>>>>  #include "standard-headers/linux/input.h"
>>>>>  #include "hw/arm/smmuv3.h"
>>>>> +#include "hw/acpi/acpi.h"
>>>>>  
>>>>>  #define DEFINE_VIRT_MACHINE_LATEST(major, minor, latest) \
>>>>>      static void virt_##major##_##minor##_class_init(ObjectClass *oc, \
>>>>> @@ -94,34 +95,25 @@
>>>>>  
>>>>>  #define PLATFORM_BUS_NUM_IRQS 64
>>>>>  
>>>>> -/* RAM limit in GB. Since VIRT_MEM starts at the 1GB mark, this means
>>>>> - * RAM can go up to the 256GB mark, leaving 256GB of the physical
>>>>> - * address space unallocated and free for future use between 256G and 
>>>>> 512G.
>>>>> - * If we need to provide more RAM to VMs in the future then we need to:
>>>>> - *  * allocate a second bank of RAM starting at 2TB and working up
>>> I acknowledge this comment was the main justification. Now if you look at
>>>
>>> Principles of ARM Memory Maps
>>> http://infocenter.arm.com/help/topic/com.arm.doc.den0001c/DEN0001C_principles_of_arm_memory_maps.pdf
>>> chapter 2.3 you will find that when adding PA bits, you always leave
>>> space for reserved space and mapped IO.
>>
>> Thanks for the pointer!
>>
>> So ... we can fit
>>
>> a) 2GB at 2GB
>> b) 32GB at 32GB
>> c) 512GB at 512GB
>> d) 8TB at 8TB
>> e) 128TB at 128TB
>>
>> (this is a nice rule of thumb if I understand it correctly :) )
>>
>> We should strive for device memory (maxram_size - ram_size) to fit
>> exactly into one of these slots (otherwise things get nasty).
>>
>> Depending on the ram_size, we might have simpler setups and can support
>> more configurations, no?
>>
>> E.g. ram_size <= 34GB, device_memory <= 512GB
>> -> move ram into a) and b)
>> -> move device memory into c)
> 
> The issue is machvirt doesn't comply with that document.
> At the moment we have
> 0 -> 1GB MMIO
> 1GB -> 256GB RAM
> 256GB -> 512GB is theoretically reserved for IO but most is free.
> 512GB -> 1T is reserved for ECAM MMIO range. This is the top of our
> existing 40b GPA space.
> 
> We don't want to change this address map due to legacy reasons.
> 

Thanks, good to know!

> Another question! do you know if it would be possible to have
> device_memory region split into several discontinuous segments?

It can be implemented for sure, but I would try to avoid that, as it
makes certain configurations impossible (and very end user unfriendly).

E.g. (numbers completely made up, but it should show what I mean)

-m 20G,maxmem=120G:
-> Try to add a DIMM with 100G -> error.
-> But we can add e.g. two DIMMs with 40G and 60G.

This exposes internal details to the end user. And the end user has no
idea what is going on.

So I think we should try our best to keep that area consecutive.

> 
> Thanks
> 
> Eric


-- 

Thanks,

David / dhildenb