Re: [PATCH v3 15/16] iommu: introduce page response function

Jean-Philippe Brucker
2017-12-05T17:17:52Z

Hi Jacob,

On 04/12/17 21:37, Jacob Pan wrote:
> On Fri, 24 Nov 2017 12:03:50 +0000
> Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:
>
>> On 17/11/17 18:55, Jacob Pan wrote:
>>> When nested translation is turned on and guest owns the
>>> first level page tables, device page request can be forwared
>>> to the guest for handling faults. As the page response returns
>>> by the guest, IOMMU driver on the host need to process the
>>> response which informs the device and completes the page request
>>> transaction.
>>>
>>> This patch introduces generic API function for page response
>>> passing from the guest or other in-kernel users. The definitions of
>>> the generic data is based on PCI ATS specification not limited to
>>> any vendor.>
>>> Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
[...]
> I think the simpler interface works for in-kernel driver use case very
> well. But in case of VFIO, the callback function does not turn around
> send back page response. The page response comes from guest and qemu,
> where they don;t keep track of the the prq event data.

Is it safe to trust whatever response the guest or userspace gives us? The
answer seems fairly vendor- and device-specific so I wonder if VFIO or
IOMMU shouldn't do a bit of sanity checking somewhere, and keep track of
all injected page requests.

>From SMMUv3 POV, it seems safe (haven't looked at SMMUv2 but I'm not so
confident).

* The guest can only send page responses to devices assigned to it, that's
a given.

* If, after we injected a page request, the guest doesn't reply at all,
then the device leaks page request credits and at some point it will
stop sending requests.
-> So the PRI capability needs to be reset whenever we change the
device's domain, to clear the credit counter and pending states.

For SMMUv3, the stall buffer may be shared between devices on some
implementations, in which case the guest could prevent other devices to
stall by letting the buffer fill up.
-> We might have to keep track of stalls in the host driver and set a
credit or timeout to each stall, if it comes to that.
-> In addition, send a terminate-all-stalls command when changing the
device's domain.

* If the guest sends spurious or duplicate page responses (where the PRGI
or PASID doesn't exist in any outstanding page request of the device)

For PRI if we send an invalid PRG Response, the endpoint sets UPRGI in
the PRI cap, and issues an Unexpected Completion. Then I suppose the
worst that happens is we get an AER report that we can't handle? I'm not
too familiar with that part of PCIe.

Stall is designed to tolerate this and will just ignore the response.

* If PRI/stall isn't even enabled, the IOMMU driver can check that in the
device configuration and not send the reply.

Regardless, I have a few comments on the page_response_msg:

> +/**
> + * Generic page response information based on PCI ATS and PASID spec.
> + * @paddr: servicing page address

Maybe call it @addr, so we don't read this field as "phys addr"

> + * @pasid: contains process address space ID, used in shared virtual memory(SVM)

The "used in shared virtual memory(SVM)" part isn't necessary and we're
changing the API name.

> + * @rid: requestor ID
> + * @did: destination device ID

I guess you can remove @rid and @did

> + * @last_req: last request in a page request group

Is @last_req needed at all, since only the last request requires a response?

> + * @resp_code: response code

The comment is missing a description for @pasid_present here

> + * @page_req_group_id: page request group index
> + * @prot: page access protection flag, e.g. IOMMU_FAULT_READ, IOMMU_FAULT_WRITE

Is @prot really needed in the response?

> + * @type: group or stream response

The page request doesn't provide this information

> + * @private_data: uniquely identify device-specific private data for an
> + * individual page response
> +
> + */
> +struct page_response_msg {
> + u64 paddr;
> + u32 pasid;
> + u32 rid:16;
> + u32 did:16;
> + u32 resp_code:4;
> + u32 last_req:1;
> + u32 pasid_present:1;
> +#define IOMMU_PAGE_RESP_SUCCESS 0
> +#define IOMMU_PAGE_RESP_INVALID 1
> +#define IOMMU_PAGE_RESP_FAILURE 0xF

Maybe move these defines closer to resp_code.
For someone not familiar with PRI, we should add some comments about those
values:

* SUCCESS: the request was paged-in successfully
* INVALID: could not page-in one or more pages in the group
* FAILURE: permanent PRI error, may disable faults in the device

> + u32 page_req_group_id : 9;
> + u32 prot;
> + enum page_response_type type;
> + u32 private_data;
> +};
> +

Thanks,
Jean