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I am assuming you are referring to XENVER_get_features which AFAICT is a stable interface. So why should we > >>> use it to describe the presence of an unstable interface? > >>> > >>> > >>> This flag needs using in Patch 10 to reject attempts to create a VM with > >>> devices when passthrough support is unavailable. > >>> > >>> > >>> May I ask why we can't rely on the hypervisor to reject the flag when suitable? > >>> > >>> > >>> Ian, for the 4.16 release, this series either needs completing with the > >>> additional flag implemented, or this patch needs reverting to avoid us > >>> shipping a broken interface. > >>> > >>> > >>> I fail to see how the interface would be broken... Would you mind to describe a bit more what could go wrong with this interface? > >> > >> > >> After chatting with Andrew on IRC, this is my understanding. > >> > >> Today if pci=[] is specified in the VM config file then > >> XEN_DOMCTL_CDF_vpci is added. If Xen doesn't support it, Xen returns > >> an error but libxl/xl won't be able to tell exactly why it fails. So xl > >> will end up printing a generic VM creation failure. Andrew would like to > >> see something like the following in libxl: > >> > >> if ( PCI_devices && !cap.vcpi ) > >> error("Sorry - PCI not supported") > >> > >> So that the user gets a clear informative error back rather than a > >> generic VM creation failure. Also, this is a requirement for the stable > >> hypercall interface. > >> > >> > >> I think that's fine and we can implement this request easily by adding > >> XEN_SYSCTL_PHYSCAP_vpci. Rahul or Bertrand, are you guys happy with > >> doing that? Otherwise I could take it on. > >> > >> > >> As a side note, given that PCI passthrough support is actually not yet > >> complete on ARM, we could even just do the following in xl/libxl: > >> > >> if ( PCI_devices ) > >> error("Sorry - PCI not supported") > >> > >> or temporarily remove XEN_DOMCTL_CDF_vpci until PCI passthrough gets > >> finalized. > >> > > As Rahul is on leave: > > I'm ok to introduce XEN_SYSCTL_PHYSCAP_vpci. I did the same for vpmu so it's ok. > > However the problem I have is about setting this cap. > > On arm it is easy as we are not supporting vpci at the moment so the cap can be set to false. > > But how to deal with this cap in x86 code? I'm not familiar with x86 so I'm asking for advice. > > As the sysctl is mainly from tool stacks to drive guests (DomU-s), I'd > set it to false for x86 as well. Roger - do you see any reason this > could be needed to express anything Dom0-related? I agree, I don't think we should set it to true unless we also support vPCI for domUs on x86, or else it's just going to confuse the toolstack. Thanks, Roger.