From: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx> Sent: Tuesday, November 15, 2022 5:25 PM > > On 11/14/2022 11:09 AM, Michael Kelley (LINUX) wrote: > > From: Wei Liu <wei.liu@xxxxxxxxxx> Sent: Monday, November 14, 2022 5:59 AM > >> > >> On Fri, Nov 11, 2022 at 02:53:59PM -0800, Nuno Das Neves wrote: > >>> On 11/11/2022 9:58 AM, Michael Kelley (LINUX) wrote: > >>>> From: Wei Liu <wei.liu@xxxxxxxxxx> Sent: Friday, November 11, 2022 9:27 AM > >> [...] > >>> > >>> I've tested this patch on these Azure SKUs: > >>> - Standard_D2S_v2 (intel xapic) > >>> - Standard_D4ds_v4 (intel xapic) > >>> - Standard_D4ds_v5 (intel x2apic) > >>> - Standard_D4ads_v5 (amd xapic) > >>> > >>> I've tested with linux Dom0 (nested hyperv root partition) and as a > >>> regular L1 guest. > >>> > >> > >> Okay. I think your tests are good. > >> > >> Michael, do you have any further concern? > >> > > > > If ms_hyperv_msi_ext_dest_id() returns "true", then > > hyperv_prepare_irq_remapping() will still return -ENODEV and you > > won't get interrupt remapping because it isn't needed, at least not > > for guest VMs. Is that what we want for the root partition? Or does > > ms_hyperv_msi_ext_dest_id() only return true in a guest partition, > > and not in the root partition? See commit d981059e13ff. > > > > I did some digging, and I *think* this function will always return "false" > in the root partition. > > The cpuids (HYPERV_CPUID_VIRT_STACK_*) that determine the result of > ms_hyperv_msi_ext_dest_id() are implemented by the virtualization stack > in Azure, so for L1 guests it depends on that. > > But, for nested root, the nested hypervisor controls which cpuids the > root partition sees, and VIRTUALIZATION_STACK_CPUID_INTERFACE is not in > that list. > > I tested this too; if I boot the kernel with an L1 guest, I can see that > the HYPERV_CPUID_VIRT_STACK_INTERFACE contains the "VS#1" signature. > If I boot as L2 Root, the signature is not present. > > I'm reasonably certain, but if I'm wrong we'll see the same breakage for > the same reason and we can fix it I guess. Sounds good. Please leave a comment somewhere in the code summarizing what you found, and stating the expectation that ms_hyperv_msi_ext_dest_id() returns "false" in the root partition. Michael
