On 20/03/2024 2:26 pm, Roger Pau Monné wrote: > On Wed, Mar 20, 2024 at 02:06:27PM +0000, Andrew Cooper wrote: >> On 20/03/2024 1:57 pm, Roger Pau Monne wrote: >>> There's no reason to force HVM guests to have a valid vcpu_info area when >>> initializing a vCPU, as the vCPU can also be brought online using the local >>> APIC, and on that path there's no requirement for vcpu_info to be setup >>> ahead >>> of the bring up. Note an HVM vCPU can operate normally without making use >>> of >>> vcpu_info. >>> >>> Restrict the check against dummy_vcpu_info to only apply to PV guests. >>> >>> Fixes: 192df6f9122d ('x86: allow HVM guests to use hypercalls to bring up >>> vCPUs') >>> Signed-off-by: Roger Pau Monné <roger....@citrix.com> >> Thanks for looking into this. But, do we actually need to force this on >> PV either? > Possibly, having now taken a look at the users it does seem they could > cope with unpopulated vcpu_info_area. > > Part of my understanding was that this was some kind of courtesy to PV > guests in order to prevent starting them without a vcpu_info, which > strictly speaking is not mandatory, but otherwise the guest vCPU won't > be able to receive interrupts, not even IPIs.
That's more of a stick than a carrot... "you must set up the area of a CPU before you can bring it online". Except as we've seen, HVM has been fine all along without it. >> The only interesting user of dummy_vcpu_info now is vcpu_info_populate() >> which can cope with any arbitrary vCPU. >> >> I have a suspicion that we can (now) delete these two checks, delete the >> dummy_vcpu_info object, and use a regular NULL pointer in >> vcpu_info_{populate,reset}(), and in doing so, remove one of the bigger >> pieces of PV-absurdity from Xen. > I was expecting this to be something we can backport. OTOH removing > the check completely (or even getting rid of dummy_vcpu_info) is not > something that we would want to backport. > > I would rather do the removal of dummy_vcpu_info as followup work. I was worried about ARM with your patch, because it's spelt HVM there, rather than PV. However, I'd forgotten that ARM's do_vcpu_op() filters ops down to just VCPUOP_register_{vcpu_info,runstate_memory_area} so VCPUOP_initialise isn't reachable. Therefore, Reviewed-by: Andrew Cooper <andrew.coop...@citrix.com> It also means ARM can't use any of the PHYS registration yet... ~Andrew