[RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier

[Jason Wang]

On 2020/6/30 下午11:39, Peter Xu wrote:
> On Tue, Jun 30, 2020 at 10:41:10AM +0800, Jason Wang wrote:
>>>       /* According to ATS spec table 2.4:
>>>        * S = 0, bits 15:12 = xxxx     range size: 4K
>>>        * S = 1, bits 15:12 = xxx0     range size: 8K
>>>        * S = 1, bits 15:12 = xx01     range size: 16K
>>>        * S = 1, bits 15:12 = x011     range size: 32K
>>>        * S = 1, bits 15:12 = 0111     range size: 64K
>>>        * ...
>>>        */
>>
>> Right, but the comment is probably misleading here, since it's for the PCI-E
>> transaction between IOMMU and device not for the device IOTLB invalidation
>> descriptor.
>>
>> For device IOTLB invalidation descriptor, spec allows a [0, ~0ULL]
>> invalidation:
>>
>> "
>>
>> 6.5.2.5 Device-TLB Invalidate Descriptor
>>
>> ...
>>
>> Size (S): The size field indicates the number of consecutive pages targeted
>> by this invalidation
>> request. If S field is zero, a single page at page address specified by
>> Address [63:12] is requested
>> to be invalidated. If S field is Set, the least significant bit in the
>> Address field with value 0b
>> indicates the invalidation address range. For example, if S field is Set and
>> Address[12] is Clear, it
>> indicates an 8KB invalidation address range with base address in Address
>> [63:13]. If S field and
>> Address[12] is Set and bit 13 is Clear, it indicates a 16KB invalidation
>> address range with base
>> address in Address [63:14], etc.
>>
>> "
>>
>> So if we receive an address whose [63] is 0 and the rest is all 1, it's then
>> a [0, ~0ULL] invalidation.
> Yes.  I think invalidating the whole range is always fine.  It's still not
> arbitrary, right?  E.g., we can't even invalidate (0x1000, 0x3000) with
> device-iotlb because of the address mask, not to say sub-pages.


Yes.


>
>>
>>>>>> How about just convert to use a range [start, end] for any notifier and move
>>>>>> the checks (e.g the assert) into the actual notifier implemented (vhost or
>>>>>> vfio)?
>>>>> IOMMUTLBEntry itself is the abstraction layer of TLB entry.  Hardware TLB entry
>>>>> is definitely not arbitrary range either (because AFAICT the hardware should
>>>>> only cache PFN rather than address, so at least PAGE_SIZE aligned).
>>>>> Introducing this flag will already make this trickier just to avoid introducing
>>>>> another similar struct to IOMMUTLBEntry, but I really don't want to make it a
>>>>> default option...  Not to mention I probably have no reason to urge the rest
>>>>> iommu notifier users (tcg, vfio) to change their existing good code to suite
>>>>> any of the backend who can cooperate with arbitrary address ranges...
>>>> Ok, so it looks like we need a dedicated notifiers to device IOTLB.
>>> Or we can also make a new flag for device iotlb just like current UNMAP? Then
>>> we replace the vhost type from UNMAP to DEVICE_IOTLB.  But IMHO using the
>>> ARBITRARY_LENGTH flag would work in a similar way.  DEVICE_IOTLB flag could
>>> also allow virtio/vhost to only receive one invalidation (now IIUC it'll
>>> receive both iotlb and device-iotlb for unmapping a page when ats=on), but then
>>> ats=on will be a must and it could break some old (misconfiged) qemu because
>>> afaict previously virtio/vhost could even work with vIOMMU (accidentally) even
>>> without ats=on.
>>
>> That's a bug and I don't think we need to workaround mis-configurated qemu
>> :)
> IMHO it depends on the strictness we want on the qemu cmdline API. :)
>
> We should at least check libvirt to make sure it's using ats=on always, then I
> agree maybe we can avoid considering the rest...
>
> Thanks,


Cc libvirt list, but I think we should fix libvirt if they don't provide 
"ats=on".

Thanks