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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.03.2024 23:08, Julien Grall wrote:
> 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?

No, I don't. I did all development with hacked up XTF tests, and I was
expecting Linux folks to be looking into making use of the new subops.

Jan

> I would like to 
> give a try on Arm before sending a patch?
> 
> Cheers,
>