On Thu, Sep 03 2020 at 00:34, Joel Fernandes wrote:
> On Wed, Sep 2, 2020 at 12:57 PM Dario Faggioli <dfaggi...@suse.com> 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
