On Thu, 17 Dec 2020 11:04:48 -0500 Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote: > On 12/17/20 7:59 AM, Cornelia Huck wrote: > > The basic question I have is whether it makes sense to specialcase the > > ISM device (can we even find out that we're dealing with an ISM device > > here?) to force the non-MIO instructions, as it is just a device with > > Yes, with the addition of the CLP data passed from the host via vfio > capabilities, we can tell this is an ISM device specifically via the > 'pft' field in VFOI_DEVICE_INFO_CAP_ZPCI_BASE. We don't actually > surface that field to the guest itself in the Q PCI FN clp rsponse (has > to do with Function Measurement Block requirements) but we can certainly > use that information in QEMU to restrict this behavior to only ISM devices. > > > some special requirements, or tie non-MIO to relaxed alignment. (Could > > relaxed alignment devices in theory be served by MIO instructions as > > well?) > > In practice, I think there are none today, but per the architecture it > IS possible to have relaxed alignment devices served by MIO > instructions, so we shouldn't rely on that bit alone as I'm doing in > this RFC. I think instead relying on the pft value as I mention above > is what we have to do. >From what you write this looks like the best way to me as well. > > > > > Another thing that came to my mind is whether we consider the guest to > > be using a pci device and needing weird instructions to do that because > > it's on s390, or whether it is issuing instructions for a device that > > happens to be a pci device (sorry if that sounds a bit meta :) > > > > Typically, I'd classify things as the former but I think ISM seems more > like the latter -- To me, ISM seems like less a classic PCI device and > more a device that happens to be using s390 PCI interfaces to accomplish > its goal. But it's probably more of a case of this particular device > (and it's driver) are s390-specific and therefore built with the unique > s390 interface in-mind (and in fact invokes it directly rather than > through the general PCI layer), rather than fitting the typical PCI > device architecture on top of the s390 interface. Nod, it certainly feels like that.
