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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

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.



-- 
Alexey