On Mon, Jan 31, 2022 at 02:56:38PM -0300, Leonardo Bras Soares Passos wrote: > Hello Daniel, > > On Mon, Jan 31, 2022 at 6:08 AM Daniel P. Berrangé <berra...@redhat.com> > wrote: > > > > CC'ing Babu Moger who aded the Milan CPU model. > > > > On Sat, Jan 29, 2022 at 07:23:37AM -0300, Leonardo Bras wrote: > > > While trying to bring a VM with EPYC-Milan cpu on a host with > > > EPYC-Milan cpu (EPYC 7313), the following warning can be seen: > > > > > > qemu-system-x86_64: warning: host doesn't support requested feature: > > > CPUID.07H:EBX.erms [bit 9] > > > qemu-system-x86_64: warning: host doesn't support requested feature: > > > CPUID.07H:EDX.fsrm [bit 4] > > > > > > Even with this warning, the host goes up. > > > > > > Then, grep'ing cpuid output on both guest and host, outputs: > > > > > > extended feature flags (7): > > > enhanced REP MOVSB/STOSB = false > > > fast short REP MOV = false > > > (simple synth) = AMD EPYC (3rd Gen) (Milan B1) [Zen 3], 7nm > > > brand = "AMD EPYC 7313 16-Core Processor " > > > > > > This means that for the same -cpu model (EPYC-Milan), the vcpu may or may > > > not have the above feature bits set, which is usually not a good idea for > > > live migration: > > > Migrating from a host with these features to a host without them can > > > be troublesome for the guest. > > > > > > Remove the "optional" features (erms, fsrm) from Epyc-Milan, in order to > > > avoid possible after-migration guest issues. > > > > Babu, can you give some insight into availability of erms / fsrm > > features across the EPYC 3rd gen CPU line. Is this example missing > > erms/fsrm an exception, or common place ? > > > > > > > > Signed-off-by: Leonardo Bras <leob...@redhat.com> > > > --- > > > > > > Does this make sense? Or maybe I am missing something here. > > > > > > Having a kvm guest running with a feature bit, while the host > > > does not support it seems to cause a possible break the guest. > > > > The guest won't see the feature bit - that warning message from QEMU > > is telling you that it did't honour the request to expose > > erms / fsrm - it has dropped them from the CPUO exposed to the guest. > > Exactly. > What I meant here is: > 1 - Host with these feature bits start a VM with EPYC-Milan cpu (and > thus have those bits enabled) > 2 - Guest is migrated to a host such as the above, which does not > support those features (bits disabled), but does support EPYC-Milan > cpus (without those features). > 3 - The migration should be allowed, given the same cpu types. Then > either we have: > 3a : The guest vcpu stays with the flag enabled (case I tried to > explain above), possibly crashing if the new feature is used, or > 3b: The guest vcpu disables the flag due to incompatibility, which > may make the guest confuse due to cpu change, and even end up trying > to use the new feature on the guest, even if it's disabled.
Neither should happen with a correctly written mgmt app in charge. When launching a QEMU process for an incoming migration, it is expected that the mgmt app has first queried QEMU on the source to get the precise CPU model + flags that were added/removed on the source. The QEMU on the target is then launched with this exact set of flags, and the 'check' flag is also set for -cpu. That will cause QEMU on the target to refuse to start unless it can give the guest the 100% identical CPUID to what has been requested on the CLI, and thus matching the source. Libvirt will ensure all this is done correctly. If not using libvirt then you've got a bunch of work to do to achieve this. It certainly isn't sufficient to merely use the same plain '-cpu' arg that the soruce was original booted with, unless you have 100% identical hardware, microcode, and software on both hosts, or the target host offers a superset of features. 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 :|
