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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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...

~Andrew