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

[Alexander Graf <agraf@suse.de>]

On 05.06.14 15:10, 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?

>
>
>> 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.


Alex