Re: [libvirt] [PATCH 2/2] qemu: Remove host-passthrough validation check for host-phys-bits=on

On 9/2/22 08:50, Dario Faggioli wrote: > On Wed, 2022-08-17 at 11:18 -0600, Jim Fehlig wrote: >> On 8/17/22 08:47, Jim Fehlig wrote: >>> On 8/17/22 02:19, Lin Ma wrote: >>>> Besides the -cpu host, The host-phys-bits=on applies to custom or >>>> max >>>> cpu model, So the host-passthrough validation check is >>>> unnecessary for >>>> maxphysaddr with mode='passthrough'. >>> >>> I wonder why Dario added the check? He even added a test for it: >>> cpu-phys-bits-passthrough2 in qemuxml2argvtest. He is off now, so >>> we'll have to >>> await his return for an answer. >> >> BTW, I'm not doubting your analysis. In fact, I tested with one of >> the known CPU >> models (-cpu Cascadelake-Server,...,host-phys-bits=on) and it worked >> fine with a >> 7TB VM. I'd still like to know why Dario added the check. Perhaps he >> encountered >> some problematic corner cases. >> > Not really, no. IIRC, QEMU was behaving differently than how it work > this days, when I wrote this, but even back then host-phys-bits=on was > fine with -cpu != host. > > The check is there just because I thought that, if you're using a > specific CPU model, and not host-passthrough, that's probably because > you don't want the host details to "leak" into the VM (for migration or > whatever). Hence, you shouldn't use phys-bits passthrough either. Hmm, an interesting point... > Like, you need the CPU to be CascadeLake-Server for migration purposes, > your host has 52 bits and you know you need a large VM so instead of > wasting time doing the math, you just go for passthrough (for the phys > bits, of course). Then you migrate on an host with 46 bits, which would > also have been fine for the size VM, but now, since you've basically > used 52, it's a problem. but at least for the migration case we have that covered by the ABI stability check https://gitlab.com/libvirt/libvirt/-/blob/master/src/conf/cpu_conf.c#L1113