Re: [RFC PATCH v7 17/23] kernel/entry: Add support for core-wide protection of kernel-mode

[Thomas Gleixner]

On Thu, Sep 03 2020 at 00:34, Joel Fernandes wrote:
> On Wed, Sep 2, 2020 at 12:57 PM Dario Faggioli <dfaggioli@xxxxxxxx> wrote:
>> 2) protection of the kernel from the other thread running in userspace
>> may be achieved in different ways. This is one, sure. ASI will probably
>> be another. Hence if/when we'll have both, this and ASI, it would be
>> cool to be able to configure the system in such a way that there is
>> only one active, to avoid paying the price of both! :-)
>
> Actually, no. Part of ASI will involve exactly what this patch does -
> IPI-pausing siblings but ASI does so when they have no choice but to
> switch away from the "limited kernel" mapping, into the full host
> kernel mapping. I am not sure if they have yet implemented that part
> but they do talk of it in [1] and in their pretty LPC slides.  It is
> just that ASI tries to avoid that scenario of kicking all siblings out
> of guest mode.  So, maybe this patch can be a stepping stone to ASI.
> At least I got the entry hooks right, and the algorithm is efficient
> IMO (useless IPIs are avoided).  ASI can then come in and avoid
> sending IPIs even more by doing their limited-kernel-mapping things if
> needed. So, it does not need to be this vs ASI, both may be needed.

Right. There are different parts which are seperate:

1) Core scheduling as a best effort feature (performance for certain use
   cases)

2) Enforced core scheduling (utilizes #1 basics)

3) ASI

4) Kick sibling out of guest/host and wait mechanics

#1, #2, #3 can be used stand alone. #4 is a utility

Then you get combos:

A) #2 + #4:

   core wide protection. i.e. what this series tries to achieve.  #3
   triggers the kick at the low level VMEXIT or entry from user mode
   boundary. The wait happens at the same level

B) #3 + #4:

   ASI plus kicking the sibling/wait mechanics independent of what's
   scheduled. #3 triggers the kick at the ASI switch to full host
   mapping boundary and the wait is probably the same as in #A

C) #2 + #3 + #4:

   The full concert, but trigger/wait wise the same as #B

So we really want to make at least #4 an independent utility.

Thanks,

        tglx