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

[Alexander Graf <agraf@suse.de>]

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).

That way you don't have to reserve a CAP today.


Alex