On Tue, May 05, 2020 at 02:02:58PM +0200, Paolo Bonzini wrote: > On 05/05/20 13:28, Kashyap Chamarthy wrote: [...] > > +Migrating an L1 guest merely configured to support nesting, while not > > +actually running L2 guests, is expected to function normally. > > +Live-migrating an L2 guest from one L1 guest to another is also expected > > +to succeed. > > + > > This is a bit optimistic, as AMD is not supported yet. Please review > the following incremental patch: Hi, Paolo; it wasn't entirely clear what you meant by "incremental patch": (a) You're going to squash it in the current commit, or (b) You're going to add it as a patch on top, or (c) I should send a v4 with your correction below Which is it? :-) (Only option-c has an action on me.) > diff --git a/Documentation/virt/kvm/running-nested-guests.rst b/Documentation/virt/kvm/running-nested-guests.rst > --- a/Documentation/virt/kvm/running-nested-guests.rst > +++ b/Documentation/virt/kvm/running-nested-guests.rst > @@ -182,11 +182,23 @@ Enabling "nested" (s390x) > Live migration with nested KVM > ------------------------------ > > -The below live migration scenarios should work as of Linux kernel 5.3 > -and QEMU 4.2.0 for x86; for s390x, even older versions might work. > -In all the below cases, L1 exposes ``/dev/kvm`` in it, i.e. the L2 guest > -is a "KVM-accelerated guest", not a "plain emulated guest" (as done by > -QEMU's TCG). > +Migrating an L1 guest, with a *live* nested guest in it, to another > +bare metal host, works as of Linux kernel 5.3 and QEMU 4.2.0 for > +Intel x86 systems, and even on older versions for s390x. > + > +On AMD systems, once an L1 guest has started an L2 guest, the L1 guest > +should no longer be migrated or saved (refer to QEMU documentation on > +"savevm"/"loadvm") until the L2 guest shuts down. Attempting to migrate > +or save-and-load an L1 guest while an L2 guest is running will result in > +undefined behavior. You might see a ``kernel BUG!`` entry in ``dmesg``, a > +kernel 'oops', or an outright kernel panic. Such a migrated or loaded L1 > +guest can no longer be considered stable or secure, and must be restarted. > +Migrating an L1 guest merely configured to support nesting, while not > +actually running L2 guests, is expected to function normally even on AMD > +systems but may fail once guests are started. > + > +Migrating an L2 guest is expected to succeed, so all the following > +scenarios should work even on AMD systems: > > - Migrating a nested guest (L2) to another L1 guest on the *same* bare > metal host. > @@ -194,30 +206,7 @@ QEMU's TCG). > - Migrating a nested guest (L2) to another L1 guest on a *different* > bare metal host. > > -- Migrating an L1 guest, with an *offline* nested guest in it, to > - another bare metal host. > - > -- Migrating an L1 guest, with a *live* nested guest in it, to another > - bare metal host. > - > -Limitations on Linux kernel versions older than 5.3 (x86) > ---------------------------------------------------------- > - > -On Linux kernel versions older than 5.3, once an L1 guest has started an > -L2 guest, the L1 guest would no longer capable of being migrated, saved, > -or loaded (refer to QEMU documentation on "save"/"load") until the L2 > -guest shuts down. > - > -Attempting to migrate or save-and-load an L1 guest while an L2 guest is > -running will result in undefined behavior. You might see a ``kernel > -BUG!`` entry in ``dmesg``, a kernel 'oops', or an outright kernel panic. > -Such a migrated or loaded L1 guest can no longer be considered stable or > -secure, and must be restarted. > - > -Migrating an L1 guest merely configured to support nesting, while not > -actually running L2 guests, is expected to function normally. > -Live-migrating an L2 guest from one L1 guest to another is also expected > -to succeed. > +- Migrating a nested guest (L2) to a bare metal host. > > Reporting bugs from nested setups > ----------------------------------- > -- /kashyap
