Re: [Xen-devel] [PATCH] xl: remove apic option for PVH guests

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/03/18 11:09, Wei Liu wrote:
> On Thu, Mar 01, 2018 at 05:01:55PM +0000, Roger Pau Monné wrote:
>> On Thu, Mar 01, 2018 at 04:01:23PM +0000, Wei Liu wrote:
>>> On Thu, Mar 01, 2018 at 03:57:18PM +0000, Andrew Cooper wrote:
>>>> On 01/03/18 12:22, Wei Liu wrote:
>>>>> On Wed, Feb 28, 2018 at 10:20:53AM +0000, Roger Pau Monne wrote:
>>>>>> XSA-256 forces the local APIC to always be enabled for PVH guests, so
>>>>>> ignore any apic option for PVH guests. Update the documentation
>>>>>> accordingly.
>>>>> I think how I will approach this is to dictate that PVH always has LAPIC
>>>>> in our in-tree document, then use that as the justification for this
>>>>> change. That's the consensus from 2 years ago, right?
>>>>>
>>>>> Or we're just working around the limitation in our code base, and users
>>>>> may demand a no-LAPIC PVH guest just because...
>>>> Currently, Xen enforces that HVM guests have an LAPIC.  This is because
>>>> making the non-LAPIC case function correctly/safely devolved into a
>>>> massive rats nest and I stopped trying to fix it after 2 days of trying.
>>>>
>>>> At the moment, it would be wise to discuss whether the non-LAPIC case is
>>>> actually sensible.  I personally see no value in keeping it.
>>>>
>>> +1
>>>
>>>> If someone can come up with a convincing usecase for keeping it, then
>>>> ok, but the barrier for this is increasing all the time, especially now
>>>> that hardware acceleration and posted interrupts means that a
>>>> pipeline-virtualised APIC is faster and more efficient than any of our
>>>> event channel mechanisms.
>>> +1
>> I've looked at the in-tree pvh document and it just refers to the local
>> APIC in this sentence:
>>
>> "AP startup can be performed using hypercalls or the local APIC if present."
>>
>> I guess the trailing "if present" could be removed, but it's not
>> colliding with this patch.
>>
>> I'm happy with rebasing this patch and applying the above change, is
>> there any other document that should be changed?
> Can we make it more explicit. Like
>
>   VCPUs for PVH must have local APIC and it can't be disabled.

-1 to this.  When an APIC is available to the guest, there is soft
disable and hard disable as part of the state model.  Saying this will
only confuse the matter.

~Andrew

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