On Thu, Jul 04, 2024 at 11:31:16AM +0200, Paolo Bonzini wrote: > On Thu, Jul 4, 2024 at 10:42 AM Daniel P. Berrangé <berrange@xxxxxxxxxx> wrote: > > > > On Thu, Jul 04, 2024 at 08:51:05AM +0200, Paolo Bonzini wrote: > > > On Thu, Jul 4, 2024 at 2:01 AM Michael Roth <michael.roth@xxxxxxx> wrote: > > > > Currently if the 'legacy-vm-type' property of the sev-guest object is > > > > left unset, QEMU will attempt to use the newer KVM_SEV_INIT2 kernel > > > > interface in conjunction with the newer KVM_X86_SEV_VM and > > > > KVM_X86_SEV_ES_VM KVM VM types. > > > > > > > > This can lead to measurement changes if, for instance, an SEV guest was > > > > created on a host that originally had an older kernel that didn't > > > > support KVM_SEV_INIT2, but is booted on the same host later on after the > > > > host kernel was upgraded. > > > > > > I think this is the right thing to do for SEV-ES. I agree that it's > > > bad to require a very new kernel (6.10 will be released only a month > > > before QEMU 9.1), on the other hand the KVM_SEV_ES_INIT API is broken > > > in several ways. As long as there is a way to go back to it, and it's > > > not changed by old machine types, not using it for SEV-ES is the > > > better choice for upstream. > > > > Broken how ? I know there was the regression with the 'debug_swap' > > parameter, but was something that should just be fixed in the kernel, > > rather than breaking userspace. What else is a problem ? > > The debug_swap parameter simply could not be enabled in the old API > without breaking measurements. The new API *is the fix* to allow using > it (though QEMU doesn't have the option plumbed in yet). There is no > extensibility. > > Enabling debug_swap by default is also a thorny problem; it cannot be > enabled by default because not all CPUs support it, and also we'd have > the same problem that we cannot enable debug_swap on new machine types > without requiring a new kernel. Tying the default to the -cpu model > would work but it is confusing. Presumably we can tie it to '-cpu host' without much problem, and then just leave it as an opt-in feature flag for named CPU models. > But I guess we can add support for debug_swap, disabled by default and > switch to the new API if debug_swap is enabled. > > > I don't think its reasonable for QEMU to require a brand new kernel > > for new machine types, given SEV & SEV-ES have been deployed for > > many years already. > > I think it's reasonable if the fix is displayed right into the error > message. It's only needed for SEV-ES though, SEV can use the old and > new APIs interchangeably. FYI currently it is proposed to unconditionally force set legacy-vm-type=true in libvirt, so QEMU guests would *never* use the new ioctl, to fix what we consider to be a QEMU / kernel guest ABI regression. If libvirt adds this though, it is basically a forever setting we would never be able to remove as removing would break ABI compat again. https://lists.libvirt.org/archives/list/devel@xxxxxxxxxxxxxxxxx/message/KP24LPFV4OCMN45TDHNXQVBPDZ56QSRR/ I'd much rather QEMU did NOT set this by default in its machine types, so libvirt doesn't have to add this forced flag. That way downstream distros who /can/ guarantee new enough kernels, can still use legacy-vm-type=false in their downstream machine types, without libvirt overriding this. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
