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

[Joel Fernandes]

On Thu, Sep 3, 2020 at 9:20 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> 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.

Agreed! Thanks for enlisting all the cases so well. I believe this
could be achieved by moving the calls to unsafe_enter() and
unsafe_exit() to when ASI decides it is time to enter the unsafe
kernel context.  I will keep it in mind when sending the next revision
as well.

thanks,

- Joel