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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.03.2024 10:10, Roger Pau Monné wrote:
> On Thu, Mar 21, 2024 at 09:07:10AM +0100, Jan Beulich wrote:
>> On 20.03.2024 17:29, Roger Pau Monné wrote:
>>> On Wed, Mar 20, 2024 at 04:09:33PM +0100, Jan Beulich wrote:
>>>> On 20.03.2024 14:57, 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.
>>>>
>>>> While I'd agree if you started with "There's no real need to force ...", I
>>>> still think there is a reason: If one wants to use paravirt interfaces 
>>>> (i.e.
>>>> hypercalls), they would better do so consistently. After all there's also
>>>> no need to use VCPUOP_initialise, yet you're not disabling its use.
>>>>
>>>> As said in reply to Andrew's reply, besides acting as a sentinel that
>>>> structure instance also acts as a sink for Xen accesses to a vCPU's
>>>> vcpu_info. By removing the check, you switch that from being a safeguard to
>>>> being something that actually has to be expected to be accessed. Unless
>>>> you've audited all uses to prove that no such access exists.
>>>
>>> I'm kind of lost in this last paragraph, how is that different than
>>> what currently happens when an HVM vCPU >= 32 is brought up using the
>>> lapic and has no vpcu_info mapped?
>>
>> I think this aspect was simply missed back at the time. And I think it
>> wants mentioning explicitly to justify the change.
> 
> OK, I can add to the commit message:
> 
> "Note an HVM vCPU can operate normally without making use of
> vcpu_info, and in fact does so when brought up from the local APIC."

I'd be fine adding this (or having this added) while committing.

>> As said in reply to Andrew, I don't think the dummy structure can be got
>> rid of. Nor can the checks here be (easily) removed altogether, i.e. your
>> change cannot (easily) be extended to PV as well. Even conditional removal
>> of the structure in !PV builds would first require all vcpu_info accesses
>> to gain a suitable conditional. Which may be undesirable, as some of these
>> may be deemed fast paths.
> 
> I didn't intended to do this here, as replied to Andrew.  If we want
> to get rid of the check for PV also it needs to be done in a different
> patch, and with a different justification and analysis.
> 
>>> Also, from a quick look it seems like sites do check whether vcpu_info
>>> == dummy_vcpu_info, otherwise we would already be in trouble.
>>
>> Such checks exist in code managing vcpu_info, but not - afaics - in places
>> actually accessing it.
> 
> Quite possibly, I didn't look that close TBH, since my intention was
> not to remove dummy_vcpu_info.  I've noticed however that
> __update_vcpu_system_time() checks for v->vcpu_info_area.map == NULL,
> which is fine, but shouldn't it also check for v->vcpu_info_area.map
> == &dummy_vcpu_info, as it's pointless to update the vcpu system time
> if pointing to the dummy_vcpu_info?

The check is there to guard against NULL deref. As said, the aspect of a
vCPU being brought up the "native" way yet then still using its vCPU info
was, by mistake, neglected earlier on. So yes, such a check could be
added here, but it isn't strictly necessary as long as we don't avoid
accessing the dummy structure uniformly everywhere (which, as said, I'm
not sure we want to do).

Jan