Re: kvm vmload/vmsave vs tss.ist

[Ingo Molnar <mingo@elte.hu>]

* Avi Kivity <avi@redhat.com> wrote:

> I would like to remove this limitation.  I see several ways to go about 
> it:
>
> 1. Drop the use of IST
>
> This would reduce the (perceived) reliability of the kernel and would 
> probably not be welcomed.

> hpa/Ingo, any opinions?

i think we should actually do #1 unconditionally.

ISTs are bad for the native kernel too. They have various nasty 
complications in the stack walker (and hence they _reduce_ reliability in 
practice), and they are non-preemptible as well. Plus we have the 
maximum-stack-footprint ftrace plugin now, which can remove any perception 
about how bad the worst-case stack footprint is in practice.

If it ever becomes an issue we could also soft-switch to a larger (per 
CPU) exception stack from the exception handlers themselves. The 
architectural stack footprint of the various critical exceptions are 
calculatable and low - so we could switch away and get almost the kind of 
separation that ISTs give. There's no deep reason to actually make use of 
hw switched ISTs.

So feel free to send a patch that just standardizes the critical 
exceptions to use the regular kernel stack. (I havent actually tried this 
but it should be relatively simple to implement. Roadblocks are possible.)

	Ingo