Re: [Qemu-devel] [PATCH v7 1/4] spapr_iommu: Make in-kernel TCE table optional

[Alexander Graf <agraf@suse.de>]

On 05.06.14 15:33, Alexey Kardashevskiy wrote:
> On 06/05/2014 11:15 PM, Alexander Graf wrote:
>> On 05.06.14 15:10, Alexey Kardashevskiy wrote:
>>> On 06/05/2014 11:06 PM, Alexander Graf wrote:
>>>> On 05.06.14 08:43, Alexey Kardashevskiy wrote:
>>>>> On 06/05/2014 03:49 PM, Alexey Kardashevskiy wrote:
>>>>>> POWER KVM supports an KVM_CAP_SPAPR_TCE capability which allows
>>>>>> allocating
>>>>>> TCE tables in the host kernel memory and handle H_PUT_TCE requests
>>>>>> targeted to specific LIOBN (logical bus number) right in the host without
>>>>>> switching to QEMU. At the moment this is used for emulated devices only
>>>>>> and the handler only puts TCE to the table. If the in-kernel H_PUT_TCE
>>>>>> handler finds a LIOBN and corresponding table, it will put a TCE to
>>>>>> the table and complete hypercall execution. The user space will not be
>>>>>> notified.
>>>>>>
>>>>>> Upcoming VFIO support is going to use the same sPAPRTCETable device class
>>>>>> so KVM_CAP_SPAPR_TCE is going to be used as well. That means that TCE
>>>>>> tables for VFIO are going to be allocated in the host as well.
>>>>>> However VFIO operates with real IOMMU tables and simple copying of
>>>>>> a TCE to the real hardware TCE table will not work as guest physical
>>>>>> to host physical address translation is requited.
>>>>>>
>>>>>> So until the host kernel gets VFIO support for H_PUT_TCE, we better not
>>>>>> to register VFIO's TCE in the host.
>>>>>>
>>>>>> This adds a bool @kvm_accel flag to the sPAPRTCETable device telling
>>>>>> that sPAPRTCETable should not try allocating TCE table in the host
>>>>>> kernel.
>>>>>> Instead, the table will be created in QEMU.
>>>>>>
>>>>>> This adds an kvm_accel parameter to spapr_tce_new_table() to let users
>>>>>> choose whether to use acceleration or not. At the moment it is enabled
>>>>>> for VIO and emulated PCI. Upcoming VFIO support will set it to false.
>>>>>>
>>>>>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>>>>>> ---
>>>>>>
>>>>>> This is a workaround but it lets me have one IOMMU device for VIO,
>>>>>> emulated
>>>>>> PCI and VFIO which is a good thing.
>>>>>>
>>>>>> The other way around would be a new KVM_CAP_SPAPR_TCE_VFIO capability but
>>>>>> this needs kernel update.
>>>>> Never mind, I'll make it a capability. I'll post capability reservation
>>>>> patch separately.
>>>> Just rename the flag from "kvm_accel" to "vfio_accel", set it to true for
>>>> vfio and false for emulated devices. Then the spapr_iommu file can check on
>>>> the capability (and default to false for now, since it doesn't exist yet).
>>> Is that ok if the flag does not have to do anything with VFIO per se? :)
>> The flag means "use in-kernel acceleration if the vfio coupling capability
>> is available", no?
> It is a flag of sPAPRTCETable which is not supposed to know about VFIO at
> all, it is just an IOMMU. But if you are ok with it, I have no reason to be
> unhappy either :)
>
>
>
>>>> That way you don't have to reserve a CAP today.
>>> Why exactly cannot we do that today?
>> Because the CAP namespace isn't a garbage bin we can just throw IDs at.
>> Maybe we realize during patch review that we need completely different CAPs.
> That was my first plan - to wait for KVM_CAP_SPAPR_TCE_64 be available in
> the kernel.

So all you need are 64bit TCEs with bus_offset? What about the missing 
in-kernel modification of the shadow TCEs on H_PUT_TCE? I thought that's 
what this is really about.


Alex