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

From: Alexey Kardashevskiy
Subject: Re: [Qemu-ppc] [PATCH v7 1/4] spapr_iommu: Make in-kernel TCE table optional
Date: Fri, 06 Jun 2014 00:33:30 +1000
User-agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 06/05/2014 11:36 PM, Alexander Graf wrote:
> 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 <address@hidden>
>>>>>>> ---
>>>>>>> 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? 

No. I need 64bit IOBAs a.k.a. PCI bus addresses. The default DMA window is
just 1 or 2GB and it is mapped at 0 on PCI bus.

TCEs are 64 bit already.

> What about the missing
> in-kernel modification of the shadow TCEs on H_PUT_TCE? I thought that's
> what this is really about.

This I do not understand :(


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