Re: [PATCH 0/8] s390x/pci: Fixing s390 vfio-pci ISM support

[Pierre Morel <pmorel@linux.ibm.com>]

On 1/19/21 9:44 PM, Matthew Rosato wrote:
> Today, ISM devices are completely disallowed for vfio-pci passthrough as
> QEMU rejects the device due to an (inappropriate) MSI-X check.  Removing
> this fence, however, reveals additional deficiencies in the s390x PCI
> interception layer that prevent ISM devices from working correctly.
> Namely, ISM block write operations have particular requirements in regards
> to the alignment, size and order of writes performed that cannot be
> guaranteed when breaking up write operations through the typical
> vfio_pci_bar_rw paths. Furthermore, ISM requires that legacy/non-MIO
> s390 PCI instructions are used, which is also not guaranteed when the I/O
> is passed through the typical userspace channels.
> 
> This patchset provides a set of fixes related to enabling ISM device
> passthrough and includes patches to enable use of a new vfio region that
> will allow s390x PCI pass-through devices to perform s390 PCI instructions
> in such a way that the same instruction issued on the guest is re-issued
> on the host.
> 
> Associated kernel patchset:
> https://lkml.org/lkml/2021/1/19/874
> 
> Changes from RFC -> v1:
> - Refresh the header sync (built using Eric's 'update-linux-headers:
> Include const.h' + manually removed pvrdma_ring.h again)
> - Remove s390x/pci: fix pcistb length (already merged)
> - Remove s390x/pci: Fix memory_region_access_valid call (already merged)
> - Fix bug: s390_pci_vfio_pcistb should use the pre-allocated PCISTB
> buffer pcistb_buf rather than allocating/freeing its own.
> - New patch: track the PFT (PCI Function Type) separately from guest CLP
> response data -- we tell the guest '0' for now due to limitations in
> measurement block support, but we can still use the real value provided via
> the vfio CLP capabilities to make decisions.
> - Use the PFT (pci function type) to determine when to use the region
> for PCISTB/PCILG (only for ISM), rather than using the relaxed alignment
> bit.
> - As a result, the pcistb_default is now updated to also handle the
> possibility of relaxed alignment via 2 new functions, pcistb_validate_write
> and pcistb_write, which serve as wrappers to the memory_region calls.
> - New patch, which partially restores the MSI-X fence for passthrough
> devices...  Could potentially be squashed with 's390x/pci: MSI-X isn't
> strictly required for passthrough' but left separately for now as I felt it
> needed a clear commit description of why we should still fence this case.
> 
Hi,

The choice of using the new VFIO region is made on the ISM PCI function 
type (PFT), which makes the patch ISM specific, why don't we use here 
the MIO bit common to any zPCI function and present in kernel to make 
the choice?

Regards,
Pierre

-- 
Pierre Morel
IBM Lab Boeblingen