Re: [PATCH RFC 1/6] i386/pc: Account IOVA reserved ranges above 4G boundary

[Joao Martins <joao.m.martins@oracle.com>]

On 6/28/21 2:25 PM, Igor Mammedov wrote:
> On Wed, 23 Jun 2021 14:07:29 +0100
> Joao Martins <joao.m.martins@oracle.com> wrote:
> 
>> On 6/23/21 1:09 PM, Igor Mammedov wrote:
>>> On Wed, 23 Jun 2021 10:51:59 +0100
>>> Joao Martins <joao.m.martins@oracle.com> wrote:
>>>   
>>>> On 6/23/21 10:03 AM, Igor Mammedov wrote:  
>>>>> On Tue, 22 Jun 2021 16:49:00 +0100
>>>>> Joao Martins <joao.m.martins@oracle.com> wrote:
>>>>>     
>>>>>> It is assumed that the whole GPA space is available to be
>>>>>> DMA addressable, within a given address space limit. Since
>>>>>> v5.4 based that is not true, and VFIO will validate whether
>>>>>> the selected IOVA is indeed valid i.e. not reserved by IOMMU
>>>>>> on behalf of some specific devices or platform-defined.
>>>>>>
>>>>>> AMD systems with an IOMMU are examples of such platforms and
>>>>>> particularly may export only these ranges as allowed:
>>>>>>
>>>>>> 	0000000000000000 - 00000000fedfffff (0      .. 3.982G)
>>>>>> 	00000000fef00000 - 000000fcffffffff (3.983G .. 1011.9G)
>>>>>> 	0000010000000000 - ffffffffffffffff (1Tb    .. 16Pb)
>>>>>>
>>>>>> We already know of accounting for the 4G hole, albeit if the
>>>>>> guest is big enough we will fail to allocate a >1010G given
>>>>>> the ~12G hole at the 1Tb boundary, reserved for HyperTransport.
>>>>>>
>>>>>> When creating the region above 4G, take into account what
>>>>>> IOVAs are allowed by defining the known allowed ranges
>>>>>> and search for the next free IOVA ranges. When finding a
>>>>>> invalid IOVA we mark them as reserved and proceed to the
>>>>>> next allowed IOVA region.
>>>>>>
>>>>>> After accounting for the 1Tb hole on AMD hosts, mtree should
>>>>>> look like:
>>>>>>
>>>>>> 0000000100000000-000000fcffffffff (prio 0, i/o):
>>>>>> 	alias ram-above-4g @pc.ram 0000000080000000-000000fc7fffffff
>>>>>> 0000010000000000-000001037fffffff (prio 0, i/o):
>>>>>> 	alias ram-above-1t @pc.ram 000000fc80000000-000000ffffffffff    
>>>>>
>>>>> You are talking here about GPA which is guest specific thing
>>>>> and then somehow it becomes tied to host. For bystanders it's
>>>>> not clear from above commit message how both are related.
>>>>> I'd add here an explicit explanation how AMD host is related GPAs
>>>>> and clarify where you are talking about guest/host side.
>>>>>     
>>>> OK, makes sense.
>>>>
>>>> Perhaps using IOVA makes it easier to understand. I said GPA because
>>>> there's an 1:1 mapping between GPA and IOVA (if you're not using vIOMMU).  
>>>
>>> IOVA may be a too broad term, maybe explain it in terms of GPA and HPA
>>> and why it does matter on each side (host/guest)
>>>   
>>
>> I used the term IOVA specially because that is applicable to Host IOVA or
>> Guest IOVA (same rules apply as this is not special cased for VMs). So,
>> regardless of whether we have guest mode page tables, or just host
>> iommu page tables, this address range should be reserved and not used.
> 
> IOVA doesn't make it any clearer, on contrary it's more confusing.
> 
> And does host's HPA matter at all? (if host's firmware isn't broken,
> it should never use nor advertise 1Tb hole). 
> So we probably talking here only about GPA only.
> 
For the case in point for the series, yes it's only GPA that we care about.

Perhaps I misunderstood your earlier comment where you said how HPAs were
affected, so I was trying to encompass the problem statement in a Guest/Host
agnostic manner by using IOVA given this is all related to IOMMU reserved ranges.
I'll stick to GPA to avoid any confusion -- as that's what matters for this series.

>>>>> also what about usecases:
>>>>>  * start QEMU with Intel cpu model on AMD host with intel's iommu    
>>>>
>>>> In principle it would be less likely to occur. But you would still need
>>>> to mark the same range as reserved. The limitation is on DMA occuring
>>>> on those IOVAs (host or guest) coinciding with that range, so you would
>>>> want to inform the guest that at least those should be avoided.
>>>>  
>>>>>  * start QEMU with AMD cpu model and AMD's iommu on Intel host    
>>>>
>>>> Here you would probably only mark the range, solely for honoring how hardware
>>>> is usually represented. But really, on Intel, nothing stops you from exposing the
>>>> aforementioned range as RAM.
>>>>  
>>>>>  * start QEMU in TCG mode on AMD host (mostly form qtest point ot view)
>>>>>     
>>>> This one is tricky. Because you can hotplug a VFIO device later on,
>>>> I opted for always marking the reserved range. If you don't use VFIO you're good, but
>>>> otherwise you would still need reserved. But I am not sure how qtest is used
>>>> today for testing huge guests.  
>>> I do not know if there are VFIO tests in qtest (probably nope, since that
>>> could require a host configured for that), but we can add a test
>>> for his memory quirk (assuming phys-bits won't get in the way)
>>>   
>>
>> 	Joao
>>
>