Re: [PATCH] x86/vcpu: relax VCPUOP_initialise restriction for non-PV vCPUs

[Julien Grall <julien@xxxxxxx>]

Hi Andrew,

On 20/03/2024 14:39, Andrew Cooper wrote:
> 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.pau@xxxxxxxxxx>
> > > 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.cooper3@xxxxxxxxxx>
>
> It also means ARM can't use any of the PHYS registration yet...
Whoops. I don't think this was intended. Jan, I don't seem to find any 
use in Linux. Do you have any patches you could share? I would like to 
give a try on Arm before sending a patch?
Cheers,

--
Julien Grall