Re: [Qemu-devel] [PATCH 4/4] s390x/pci: add iommu replay callback

[Yi Min Zhao <zyimin@linux.vnet.ibm.com>]

在 2017/8/29 下午4:07, Cornelia Huck 写道:
> [Restored cc:s. Please remember to do reply-all.]
>
> On Tue, 29 Aug 2017 12:46:51 +0800
> Yi Min Zhao <zyimin@linux.vnet.ibm.com> wrote:
>
>> 在 2017/8/28 下午11:57, Cornelia Huck 写道:
>>> On Mon, 28 Aug 2017 10:04:47 +0200
>>> Yi Min Zhao <zyimin@linux.vnet.ibm.com> wrote:
>>>
>>>> Let's introduce iommu replay callback for s390 pci iommu memory region.
>>>> Currently we don't need any dma mapping replay. So let it return
>>>> directly. This implementation will avoid meaningless loops calling
>>>> translation callback.
>>>>
>>>> Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
>>>> Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
>>>> Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
>>>> ---
>>>>    hw/s390x/s390-pci-bus.c | 8 ++++++++
>>>>    1 file changed, 8 insertions(+)
>>>>
>>>> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
>>>> index 9e1f7ff5c5..359509ccea 100644
>>>> --- a/hw/s390x/s390-pci-bus.c
>>>> +++ b/hw/s390x/s390-pci-bus.c
>>>> @@ -407,6 +407,13 @@ static IOMMUTLBEntry s390_translate_iommu(IOMMUMemoryRegion *mr, hwaddr addr,
>>>>        return ret;
>>>>    }
>>>>    
>>>> +static void s390_pci_iommu_replay(IOMMUMemoryRegion *iommu,
>>>> +                                  IOMMUNotifier *notifier)
>>>> +{
>>>> +    /* we don't need iommu replay currently */
>>> This really needs to be heavier on the _why_. My guess is that anything
>>> which would require replay goes through the rpcit instruction?
>> My understanding is:
>> Our arch is different from others. Each pci device has its own iommu, not
>> like other archs' implementation. So currently there must be no existing
>> mappings belonging to any newly plugged pci device whose iommu doesn't
>> have any mapping at the time.
> So please put that explanation into the function. (Also, "currently"?
> Are we expecting it to change?)
The iommu replay function is originally introduced for vfio. I think 
they want to re-build
the existing mappings because vfio has a copy of mappings in kernel. For 
our case,
the mappings would be cleanup when a pci device unplugged, and new mappings
would be created when a pci device plugged. I think replay only can 
happen during
vfio-pci device migration.
>
>> In addition, it's also due to vfio blocking migration. If vfio-pci supports
>> migration in future, we could implement iommu replay by that time.
> That's not an argument: This is the base zpci support, it should not
> depend on the supported devices and what they do. (What's the status of
> virtio-pci, btw? Does it work with your patches applied, or is there
> still more to be done?)
>
>
My understanding is virtio-pci doesn't need replay. All mappings of any 
pci device are existing in
guest memory. Once the mappings is built, all of them could be migrated 
along with the guest
system. But I might misunderstand it. Please correct me.