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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

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.

~Andrew

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