Re: [PATCH] VMX: use a single, global APIC access page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.02.2021 18:00, Andrew Cooper wrote:
> On 10/02/2021 16:48, Jan Beulich wrote:
>> The address of this page is used by the CPU only to recognize when to
>> instead access the virtual APIC page instead. No accesses would ever go
>> to this page. It only needs to be present in the (CPU) page tables so
>> that address translation will produce its address as result for
>> respective accesses.
>>
>> By making this page global, we also eliminate the need to refcount it,
>> or to assign it to any domain in the first place.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> How certain are you about this?

The documentation (I'm inclined to say: unexpectedly) is very
clear about this; I don't think it had been this clear back at
the time. I'm hoping for Kevin to shout if he's aware of issues
here.

I've also gone through many of our emulation code paths (I
think I caught all relevant ones), and found them all having
suitable guards in place. (This process was why took until the
evening to have a patch ready.)

Jan