Re: [PATCH] KVM: Improvements for task switching

[Bernhard Kohl <bernhard.kohl@nsn.com>]

Jan Kiszka <jan.kiszka <at> siemens.com> writes:

> 
> Bernhard Kohl wrote:
> > NSN's proprietary OS DMX sometimes does task switches.
> > To get it running in KVM the following changes were necessary:
> > Interrupt injection only with interrupt flag set.
> > Linking the tss->prev_task_link to itself removed.
> > Task linking is required for CALL and GATE.
> > Do not call skip_emulated_instruction() for GATE.
> 
> Please post independent changes as separate patches. I guess the task
> linking changes belong together, but surely not to the IRQ injection
> patch. And the last change looks independent, too.

>From my point of view it is one patch. The DMX OS crashed during its task
switch. After fixing the first problem we got the 2nd, then the 3rd and 4th.
It can only complete a complete task switch with all this fixed. Obviously
all other guests don't do this kind of task switches.

> 
> Another wish (specifically as this is tricky stuff): also describe in
> the commit log, why you changed something.

OK, I will do that.

> 
> That causes concerns on my side as we had a hard time stabilizing this
> code. Need to think about it. Do you happen to have a test case for this
> (if it's not publicly shareable, contact me directly)? Did you check
> that this change causes no obvious regressions to other guests? What
> about the user-inject IRQ case, does it already work for you as-is?

The test case is our DMX OS (no public availability). Without these changes it
crashes the VM. No known other problems. Linux guests run well with these
changes. Others not tested.

> 
> What about 16-bit switches, are they already correct?

Maybe similar changes are needed for 16-bit switches. DMX does not do that.
So I have no guest to test this.

Bernhard