Re: [PATCH v2 4/4] vfio: vfio_iommu_type1: implement VFIO_IOMMU_INFO_CAPABILITIES

From: Pierre Morel
Date: Tue May 21 2019 - 08:48:49 EST


On 21/05/2019 13:11, Cornelia Huck wrote:
On Tue, 21 May 2019 11:14:38 +0200
Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:

1) A short description, of zPCI functions and groups

IN Z, PCI cards, leave behind an adapter between subchannels and PCI.
We access PCI cards through 2 ways:
- dedicated PCI instructions (pci_load/pci_store/pci/store_block)
- DMA

Quick question: What about the new pci instructions? Anything that
needs to be considered there?

No and yes.

No because they should be used when pci_{load,stor,store_block} are interpreted AFAIU.
And currently we only use interception.

Yes, because, the CLP part, use to setup the translations IIUC, (do not ask me for details now), will need to be re-issued by the kernel after some modifications and this will also need a way from QEMU S390 PCI down to the ZPCI driver.
Way that I try to setup with this patch.

So answer is not now but we should keep in mind that we will definitively need a way down to the zpci low level in the host.


We receive events through
- Adapter interrupts

Note for the non-s390 folks: These are (I/O) interrupts that are not
tied to a specific device. MSI-X is mapped to this.

- CHSC events

Another note for the non-s390 folks: This is a notification mechanism
that is using machine check interrupts; more information is retrieved
via a special instruction (chsc).


thanks, it is yes better to explain better :)


The adapter propose an IOMMU to protect the DMA
and the interrupt handling goes through a MSIX like interface handled by
the adapter.

The architecture specific PCI do the interface between the standard PCI
level and the zPCI function (PCI + DMA/IOMMU/Interrupt)

To handle the communication through the "zPCI way" the CLP interface
provides instructions to retrieve informations from the adapters.

There are different group of functions having same functionalities.

clp_list give us a list from zPCI functions
clp_query_pci_function returns informations specific to a function
clp_query_group returns information on a function group


2) Why do we need it in the guest

We need to provide the guest with information on the adapters and zPCI
functions returned by the clp_query instruction so that the guest's
driver gets the right information on how the way to the zPCI function
has been built in the host.


When a guest issues the CLP instructions we intercept the clp command in
QEMU and we need to feed the response with the right values for the guest.
The "right" values are not the raw CLP response values:

- some identifier must be virtualized, like UID and FID,

- some values must match what the host received from the CLP response,
like the size of the transmited blocks, the DMA Address Space Mask,
number of interrupt, MSIA

- some other must match what the host handled with the adapter and
function, the start and end of DMA,

- some what the host IOMMU driver supports (frame size),



3) We have three different way to get This information:

The PCI Linux interface is a standard PCI interface and some Z specific
information is available in sysfs.
Not all the information needed to be returned inside the CLP response is
available.
So we can not use the sysfs interface to get all the information.

There is a CLP ioctl interface but this interface is not secure in that
it returns the information for all adapters in the system.

The VFIO interface offers the advantage to point to a single PCI
function, so more secure than the clp ioctl interface.
Coupled with the s390_iommu we get access to the zPCI CLP instruction
and to the values handled by the zPCI driver.


4) Until now we used to fill the CLP response to the guest inside QEMU
with fixed values corresponding to the only PCI card we supported.
To support new cards we need to get the right values from the kernel out.

IIRC, the current code fills in values that make sense for one specific
type of card only, right?

yes, right

We also use the same values for emulated
cards (virtio); I assume that they are not completely weird for that
case...


No they are not.

For emulated cards, all is done inside QEMU, we do not need kernel access, the emulated cards get a specific emulation function and group assigned with pre-defined values.

I sent a QEMU patch related to this.
Even the kernel interface will change with the changes in the kernel patch, the emulation should continue in this way.

Regards,
Pierre










--
Pierre Morel
Linux/KVM/QEMU in BÃblingen - Germany