Re: [PATCH 0/2] virtio: decouple protected guest RAM form VIRTIO_F_IOMMU_PLATFORM

[Jason Wang <jasowang@redhat.com>]

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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@redhat.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@redhat.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@redhat.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
>>

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From b8a8b582f46bb86c7a745b272db7b744779e5cc7 Mon Sep 17 00:00:00 2001
From: Jason Wang <jasowang@redhat.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@redhat.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);
+    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);
         vdev->dma_as = &address_space_memory;
     }
 }
-- 
2.19.1