Date:   Mon, 24 Feb 2020 17:26:20 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>, linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Viktor Mihajlovski <mihajlov@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Michael Mueller <mimu@...ux.ibm.com>
Subject: Re: [PATCH 0/2] virtio: decouple protected guest RAM form
 VIRTIO_F_IOMMU_PLATFORM


On 2020/2/24 下午3:48, Michael S. Tsirkin wrote:
> On Mon, Feb 24, 2020 at 02:45:03PM +0800, Jason Wang wrote:
>> On 2020/2/24 下午2:06, Michael S. Tsirkin wrote:
>>> On Mon, Feb 24, 2020 at 12:01:57PM +0800, Jason Wang wrote:
>>>> On 2020/2/21 下午10:56, Halil Pasic wrote:
>>>>> On Fri, 21 Feb 2020 14:22:26 +0800
>>>>> Jason Wang <jasowang@...hat.com> wrote:
>>>>>
>>>>>> On 2020/2/21 上午12:06, Halil Pasic wrote:
>>>>>>> Currently if one intends to run a memory protection enabled VM with
>>>>>>> virtio devices and linux as the guest OS, one needs to specify the
>>>>>>> VIRTIO_F_IOMMU_PLATFORM flag for each virtio device to make the guest
>>>>>>> linux use the DMA API, which in turn handles the memory
>>>>>>> encryption/protection stuff if the guest decides to turn itself into
>>>>>>> a protected one. This however makes no sense due to multiple reasons:
>>>>>>> * The device is not changed by the fact that the guest RAM is
>>>>>>> protected. The so called IOMMU bypass quirk is not affected.
>>>>>>> * This usage is not congruent with  standardised semantics of
>>>>>>> VIRTIO_F_IOMMU_PLATFORM. Guest memory protected is an orthogonal reason
>>>>>>> for using DMA API in virtio (orthogonal with respect to what is
>>>>>>> expressed by VIRTIO_F_IOMMU_PLATFORM).
>>>>>>>
>>>>>>> This series aims to decouple 'have to use DMA API because my (guest) RAM
>>>>>>> is protected' and 'have to use DMA API because the device told me
>>>>>>> VIRTIO_F_IOMMU_PLATFORM'.
>>>>>>>
>>>>>>> Please find more detailed explanations about the conceptual aspects in
>>>>>>> the individual patches. There is however also a very practical problem
>>>>>>> that is addressed by this series.
>>>>>>>
>>>>>>> For vhost-net the feature VIRTIO_F_IOMMU_PLATFORM has the following side
>>>>>>> effect The vhost code assumes it the addresses on the virtio descriptor
>>>>>>> ring are not guest physical addresses but iova's, and insists on doing a
>>>>>>> translation of these regardless of what transport is used (e.g. whether
>>>>>>> we emulate a PCI or a CCW device). (For details see commit 6b1e6cc7855b
>>>>>>> "vhost: new device IOTLB API".) On s390 this results in severe
>>>>>>> performance degradation (c.a. factor 10).
>>>>>> Do you see a consistent degradation on the performance, or it only
>>>>>> happen when for during the beginning of the test?
>>>>>>
>>>>> AFAIK the degradation is consistent.
>>>>>
>>>>>>> BTW with ccw I/O there is
>>>>>>> (architecturally) no IOMMU, so the whole address translation makes no
>>>>>>> sense in the context of virtio-ccw.
>>>>>> I suspect we can do optimization in qemu side.
>>>>>>
>>>>>> E.g send memtable entry via IOTLB API when vIOMMU is not enabled.
>>>>>>
>>>>>> If this makes sense, I can draft patch to see if there's any difference.
>>>>> Frankly I would prefer to avoid IOVAs on the descriptor ring (and the
>>>>> then necessary translation) for virtio-ccw altogether. But Michael
>>>>> voiced his opinion that we should mandate F_IOMMU_PLATFORM for devices
>>>>> that could be used with guests running in protected mode. I don't share
>>>>> his opinion, but that's an ongoing discussion.
>>>>>
>>>>> Should we end up having to do translation from IOVA in vhost, we are
>>>>> very interested in that translation being fast and efficient.
>>>>>
>>>>> In that sense we would be very happy to test any optimization that aim
>>>>> into that direction.
>>>>>
>>>>> Thank you very much for your input!
>>>> Using IOTLB API on platform without IOMMU support is not intended. Please
>>>> try the attached patch to see if it helps.
>>>>
>>>> Thanks
>>>>
>>>>
>>>>> Regards,
>>>>> Halil
>>>>>
>>>>>> Thanks
>>>>>>
>>>>>>
>>>>>>> Halil Pasic (2):
>>>>>>>       mm: move force_dma_unencrypted() to mem_encrypt.h
>>>>>>>       virtio: let virtio use DMA API when guest RAM is protected
>>>>>>>
>>>>>>>      drivers/virtio/virtio_ring.c |  3 +++
>>>>>>>      include/linux/dma-direct.h   |  9 ---------
>>>>>>>      include/linux/mem_encrypt.h  | 10 ++++++++++
>>>>>>>      3 files changed, 13 insertions(+), 9 deletions(-)
>>>>>>>
>>>>>>>
>>>>>>> base-commit: ca7e1fd1026c5af6a533b4b5447e1d2f153e28f2
>>>> >From 66fa730460875ac99e81d7db2334cd16bb1d2b27 Mon Sep 17 00:00:00 2001
>>>> From: Jason Wang <jasowang@...hat.com>
>>>> Date: Mon, 24 Feb 2020 12:00:10 +0800
>>>> Subject: [PATCH] virtio: turn on IOMMU_PLATFORM properly
>>>>
>>>> When transport does not support IOMMU, we should clear IOMMU_PLATFORM
>>>> even if the device and vhost claims to support that. This help to
>>>> avoid the performance overhead caused by unnecessary IOTLB miss/update
>>>> transactions on such platform.
>>>>
>>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>>> ---
>>>>    hw/virtio/virtio-bus.c | 6 +++---
>>>>    1 file changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
>>>> index d6332d45c3..2741b9fdd2 100644
>>>> --- a/hw/virtio/virtio-bus.c
>>>> +++ b/hw/virtio/virtio-bus.c
>>>> @@ -47,7 +47,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
>>>>        VirtioBusState *bus = VIRTIO_BUS(qbus);
>>>>        VirtioBusClass *klass = VIRTIO_BUS_GET_CLASS(bus);
>>>>        VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
>>>> -    bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
>>>>        Error *local_err = NULL;
>>>>        DPRINTF("%s: plug device.\n", qbus->name);
>>>> @@ -77,10 +76,11 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
>>>>            return;
>>>>        }
>>>> -    if (klass->get_dma_as != NULL && has_iommu) {
>>>> -        virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
>>>> +    if (false && klass->get_dma_as != NULL &&
>>>> +        virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
>>>>            vdev->dma_as = klass->get_dma_as(qbus->parent);
>>>>        } else {
>>>> +        virtio_clear_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
>>>>            vdev->dma_as = &address_space_memory;
>>>>        }
>>>>    }
>>> This seems to clear it unconditionally. I guess it's just a debugging
>>> patch, the real one will come later?
>>
>> My bad, here's the correct one.
>>
>> Thanks
>>
>>
>>>> -- 
>>>> 2.19.1
>>>>
>> >From b8a8b582f46bb86c7a745b272db7b744779e5cc7 Mon Sep 17 00:00:00 2001
>> From: Jason Wang <jasowang@...hat.com>
>> Date: Mon, 24 Feb 2020 12:00:10 +0800
>> Subject: [PATCH] virtio: turn on IOMMU_PLATFORM properly
>>
>> When transport does not support IOMMU, we should clear IOMMU_PLATFORM
>> even if the device and vhost claims to support that. This help to
>> avoid the performance overhead caused by unnecessary IOTLB miss/update
>> transactions on such platform.
>>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   hw/virtio/virtio-bus.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
>> index d6332d45c3..4be64e193e 100644
>> --- a/hw/virtio/virtio-bus.c
>> +++ b/hw/virtio/virtio-bus.c
>> @@ -47,7 +47,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
>>       VirtioBusState *bus = VIRTIO_BUS(qbus);
>>       VirtioBusClass *klass = VIRTIO_BUS_GET_CLASS(bus);
>>       VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
>> -    bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
>>       Error *local_err = NULL;
>>   
>>       DPRINTF("%s: plug device.\n", qbus->name);
>> @@ -77,10 +76,11 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
>>           return;
>>       }
>>   
>> -    if (klass->get_dma_as != NULL && has_iommu) {
>> -        virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
> So it looks like this line is unnecessary, but it's an unrelated
> cleanup, right?


Yes.


>
>> +    if (klass->get_dma_as != NULL &&
>> +        virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
>>           vdev->dma_as = klass->get_dma_as(qbus->parent);
>>       } else {
>> +        virtio_clear_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
>
> Of course any change like that will have to affect migration compat, etc.
> Can't we clear the bit when we are sending the features to vhost
> instead?


That's better.

How about attached?

Thanks


>
>
>>           vdev->dma_as = &address_space_memory;
>>       }
>>   }
>> -- 
>> 2.19.1
>>

View attachment "0001-vhost-do-not-set-VIRTIO_F_IOMMU_PLATFORM-when-IOMMU-.patch" of type "text/x-patch" (1643 bytes)

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