Re: [EXT] Re: [PATCH v5] iommu/virtio: Use page size bitmap supported by endpoint

[Auger Eric]

Hi,

On 5/7/20 1:32 PM, Michael S. Tsirkin wrote:
> On Thu, May 07, 2020 at 11:24:29AM +0000, Bharat Bhushan wrote:
>>
>>
>>> -----Original Message-----
>>> From: Michael S. Tsirkin <mst@xxxxxxxxxx>
>>> Sent: Wednesday, May 6, 2020 5:53 AM
>>> To: Bharat Bhushan <bbhushan2@xxxxxxxxxxx>
>>> Cc: jean-philippe@xxxxxxxxxx; joro@xxxxxxxxxx; jasowang@xxxxxxxxxx;
>>> virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx;
>>> linux-kernel@xxxxxxxxxxxxxxx; eric.auger.pro@xxxxxxxxx; eric.auger@xxxxxxxxxx
>>> Subject: [EXT] Re: [PATCH v5] iommu/virtio: Use page size bitmap supported by
>>> endpoint
>>>
>>> External Email
>>>
>>> ----------------------------------------------------------------------
>>> On Tue, May 05, 2020 at 03:00:04PM +0530, Bharat Bhushan wrote:
>>>> Different endpoint can support different page size, probe endpoint if
>>>> it supports specific page size otherwise use global page sizes.
>>>>
>>>> Signed-off-by: Bharat Bhushan <bbhushan2@xxxxxxxxxxx>
>>>> ---
>>>> v4->v5:
>>>>  - Rebase to Linux v5.7-rc4
>>>>
>>>> v3->v4:
>>>>  - Fix whitespace error
>>>>
>>>> v2->v3:
>>>>  - Fixed error return for incompatible endpoint
>>>>  - __u64 changed to __le64 in header file
>>>>
>>>>  drivers/iommu/virtio-iommu.c      | 48 ++++++++++++++++++++++++++++---
>>>>  include/uapi/linux/virtio_iommu.h |  7 +++++
>>>>  2 files changed, 51 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/iommu/virtio-iommu.c
>>>> b/drivers/iommu/virtio-iommu.c index d5cac4f46ca5..9513d2ab819e 100644
>>>> --- a/drivers/iommu/virtio-iommu.c
>>>> +++ b/drivers/iommu/virtio-iommu.c
>>>> @@ -78,6 +78,7 @@ struct viommu_endpoint {
>>>>  	struct viommu_dev		*viommu;
>>>>  	struct viommu_domain		*vdomain;
>>>>  	struct list_head		resv_regions;
>>>> +	u64				pgsize_bitmap;
>>>>  };
>>>>
>>>>  struct viommu_request {
>>>> @@ -415,6 +416,19 @@ static int viommu_replay_mappings(struct
>>> viommu_domain *vdomain)
>>>>  	return ret;
>>>>  }
>>>>
>>>> +static int viommu_set_pgsize_bitmap(struct viommu_endpoint *vdev,
>>>> +				    struct virtio_iommu_probe_pgsize_mask *mask,
>>>> +				    size_t len)
>>>> +{
>>>> +	u64 pgsize_bitmap = le64_to_cpu(mask->pgsize_bitmap);
>>>> +
>>>> +	if (len < sizeof(*mask))
>>>
>>> This is too late to validate length, you have dereferenced it already.
>>> do it before the read pls.
>>
>> Yes, Will change here and other places as well
>>
>>>
>>>> +		return -EINVAL;
>>>
>>> OK but note that guest will then just proceed to ignore the property. Is that really
>>> OK? Wouldn't host want to know?
>>
>>
>> Guest need to be in sync with device, so yes seems like guest need to tell device which page-size-mask it is using.
>>
>> Corresponding spec change patch (https://www.mail-archive.com/virtio-dev@xxxxxxxxxxxxxxxxxxxx/msg06214.html)
>>
>> Would like Jean/Eric to comment here as well.
>>
>>>
>>>
>>>> +
>>>> +	vdev->pgsize_bitmap = pgsize_bitmap;
>>>
>>> what if bitmap is 0? Is that a valid size? I see a bunch of BUG_ON with that value ...
>>
>> As per spec proposed device is supposed to set at-least one bit.
>> Will add a bug_on her.
> 
> Or better fail probe ...
Yes I agree I would rather fail the probe.
> 
>> Should we add bug_on or switch to global config page-size mask if this is zero (notify device which page-size-mask it is using).
> 
> It's a spec violation, I wouldn't try to use the device.
> 
>>>
>>> I also see a bunch of code like e.g. this:
>>>
>>>         pg_size = 1UL << __ffs(pgsize_bitmap);
>>>
>>> which probably won't DTRT on a 32 bit guest if the bitmap has bits set in the high
>>> word.
>>>
>>
>> My thought is that in that case viommu_domain_finalise() will fail, do not proceed.
> 
> That's undefined behaviour in C. You need to make sure this condition
> is never reached. And spec does not make this illegal at all
> so it looks like we actually need to handle this gracefully.
> 
> 
>>>
>>>
>>>> +	return 0;
>>>> +}
>>>> +
>>>>  static int viommu_add_resv_mem(struct viommu_endpoint *vdev,
>>>>  			       struct virtio_iommu_probe_resv_mem *mem,
>>>>  			       size_t len)
>>>> @@ -499,6 +513,9 @@ static int viommu_probe_endpoint(struct viommu_dev
>>> *viommu, struct device *dev)
>>>>  		case VIRTIO_IOMMU_PROBE_T_RESV_MEM:
>>>>  			ret = viommu_add_resv_mem(vdev, (void *)prop, len);
>>>>  			break;
>>>> +		case VIRTIO_IOMMU_PROBE_T_PAGE_SIZE_MASK:
>>>> +			ret = viommu_set_pgsize_bitmap(vdev, (void *)prop, len);
>>>> +			break;
>>>>  		default:
>>>>  			dev_err(dev, "unknown viommu prop 0x%x\n", type);
>>>>  		}
>>>> @@ -630,7 +647,7 @@ static int viommu_domain_finalise(struct
>>>> viommu_endpoint *vdev,
>>>>
>>>>  	vdomain->id		= (unsigned int)ret;
>>>>
>>>> -	domain->pgsize_bitmap	= viommu->pgsize_bitmap;
>>>> +	domain->pgsize_bitmap	= vdev->pgsize_bitmap;
>>>>  	domain->geometry	= viommu->geometry;
>>>>
>>>>  	vdomain->map_flags	= viommu->map_flags;
>>>> @@ -654,6 +671,29 @@ static void viommu_domain_free(struct iommu_domain
>>> *domain)
>>>>  	kfree(vdomain);
>>>>  }
>>>>
>>>> +/*
>>>> + * Check whether the endpoint's capabilities are compatible with
>>>> +other
>>>> + * endpoints in the domain. Report any inconsistency.
>>>> + */
>>>> +static bool viommu_endpoint_is_compatible(struct viommu_endpoint *vdev,
>>>> +					  struct viommu_domain *vdomain) {
>>>> +	struct device *dev = vdev->dev;
>>>> +
>>>> +	if (vdomain->viommu != vdev->viommu) {
>>>> +		dev_err(dev, "cannot attach to foreign vIOMMU\n");
>>>> +		return false;
>>>> +	}
>>>> +
>>>> +	if (vdomain->domain.pgsize_bitmap != vdev->pgsize_bitmap) {
>>>> +		dev_err(dev, "incompatible domain bitmap 0x%lx != 0x%llx\n",
>>>> +			vdomain->domain.pgsize_bitmap, vdev->pgsize_bitmap);
>>>> +		return false;
>>>> +	}
>>>
>>> I'm confused by this. So let's assume host supports pages sizes of 4k, 2M, 1G. It
>>> signals this in the properties. Nice.
>>> Now domain supports 4k, 2M and that's all. Why is that a problem?
>>> Just don't use 1G ...
>>
>> Is not it too to change the existing domain properties, for devices already attached to domain? New devices must match to domain page-size.
> 
> Again if IOMMU supports more page sizes than domain uses, why is
> that a problem? Just don't utilize the bits domain does not use.

I think I agree with you in that case. However it is a problem in the
opposite, ie. when a new device is added and this latter has less
options than the existing domain, right?

Thanks

Eric
> 
> 
>>>
>>>
>>>> +
>>>> +	return true;
>>>> +}
>>>> +
>>>>  static int viommu_attach_dev(struct iommu_domain *domain, struct
>>>> device *dev)  {
>>>>  	int i;
>>>> @@ -670,9 +710,8 @@ static int viommu_attach_dev(struct iommu_domain
>>> *domain, struct device *dev)
>>>>  		 * owns it.
>>>>  		 */
>>>>  		ret = viommu_domain_finalise(vdev, domain);
>>>> -	} else if (vdomain->viommu != vdev->viommu) {
>>>> -		dev_err(dev, "cannot attach to foreign vIOMMU\n");
>>>> -		ret = -EXDEV;
>>>> +	} else if (!viommu_endpoint_is_compatible(vdev, vdomain)) {
>>>> +		ret = -EINVAL;
>>>>  	}
>>>>  	mutex_unlock(&vdomain->mutex);
>>>>
>>>> @@ -886,6 +925,7 @@ static int viommu_add_device(struct device *dev)
>>>>
>>>>  	vdev->dev = dev;
>>>>  	vdev->viommu = viommu;
>>>> +	vdev->pgsize_bitmap = viommu->pgsize_bitmap;
>>>>  	INIT_LIST_HEAD(&vdev->resv_regions);
>>>>  	dev_iommu_priv_set(dev, vdev);
>>>>
>>>> diff --git a/include/uapi/linux/virtio_iommu.h
>>>> b/include/uapi/linux/virtio_iommu.h
>>>> index 48e3c29223b5..2cced7accc99 100644
>>>> --- a/include/uapi/linux/virtio_iommu.h
>>>> +++ b/include/uapi/linux/virtio_iommu.h
>>>
>>> As any virtio UAPI change, you need to copy virtio TC at some point before this is
>>> merged ...
>>
>> Jean already send patch for same
>> https://www.mail-archive.com/virtio-dev@xxxxxxxxxxxxxxxxxxxx/msg06214.html
>>
>> Do we need to do anything additional?
> 
> 
> Yes, that is spec patch. you need to see the UAPI patch to virtio-dev.
> 
>>>
>>>> @@ -111,6 +111,7 @@ struct virtio_iommu_req_unmap {
>>>>
>>>>  #define VIRTIO_IOMMU_PROBE_T_NONE		0
>>>>  #define VIRTIO_IOMMU_PROBE_T_RESV_MEM		1
>>>> +#define VIRTIO_IOMMU_PROBE_T_PAGE_SIZE_MASK	2
>>>>
>>>>  #define VIRTIO_IOMMU_PROBE_T_MASK		0xfff
>>>>
>>>
>>> Does host need to know that guest will ignore the page size mask?
>>> Maybe we need a feature bit.
>>>
>>>> @@ -119,6 +120,12 @@ struct virtio_iommu_probe_property {
>>>>  	__le16					length;
>>>>  };
>>>>
>>>> +struct virtio_iommu_probe_pgsize_mask {
>>>> +	struct virtio_iommu_probe_property	head;
>>>> +	__u8					reserved[4];
>>>> +	__le64					pgsize_bitmap;
>>>> +};
>>>> +
>>>
>>> This is UAPI. Document the format of pgsize_bitmap please.
>>
>> Ok,
>>
>> Thanks
>> -Bharat
>>
>>>
>>>
>>>>  #define VIRTIO_IOMMU_RESV_MEM_T_RESERVED	0
>>>>  #define VIRTIO_IOMMU_RESV_MEM_T_MSI		1
>>>>
>>>> --
>>>> 2.17.1
>