On Thu, Oct 29, 2020 at 03:55:30PM +0000, Dario Faggioli wrote: > Hello everyone, > > These patches let one specify how wide the physical addresses are, in > bits. > > Using current QEMU's default of 40 may limit the amount of RAM the guest > can see (which is the reason why, in our packages, we bump that to 42; and > as far as I've understood from reading some old mailing list threads on > the subject, other downstreams do something similar).
In RHEL we provide custom machine types which set host-phys-bits=on by default, which is why making it configurable in libvirt hasn't been a high priority previously. > > It also can cause other problems, such as the one described in: > > https://bugzilla.redhat.com/show_bug.cgi?id=1578278#c5 > > Basically, the VM thinking and reporting to the user that L1TF is > unmitigated, because while its RAM may fits in MAX_PHYS_ADDR (e.g., equal > to 42 or 40) it does not fit in MAX_PHYS_ADDR/2, which is necessary for > PTE inversion to be effective. Yep, that's not nice. > The series alleviates the problem by providing an user with an easy way > to either specify an arbitrary number of physical address bits bits for > the VM (with, e.g., <maxphysaddr mode='emulate' bits='42'/>) or just > using the same number of bits of the host (with <maxphysaddr > mode='passthrough'/>). > > This in theory is already possible, but only in an hack-ish way, such as > adding: > > <qemu:commandline> > <qemu:arg value='-cpu'/> > <qemu:arg value='host,host-phys-bits=on'/> > </qemu:commandline> > > But this is super inconvenient. :-) And explicitly unsupported in a production deployment, so not a viable solution except for experimentation. > I have not done it such as host-phys-bits=on is automatically added when > using cpu-passthrough as CPU model, as I think that that actually > belongs in QEMU. Agreed, I think I raised that as a suggestion before too. 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 :|
