Re: SVM: vmload/vmsave-free VM exits?

[Avi Kivity <avi.kivity@gmail.com>]

On 04/13/2015 08:57 PM, Jan Kiszka wrote:
> On 2015-04-13 19:48, Avi Kivity wrote:
>> I think that Xen does (or did) something along the lines of disabling
>> IST usage (by playing with the descriptors in the IDT) and then
>> re-enabling them when exiting to userspace.
> So we would reuse that active stack for the current IST users until
> then.

Yes.

> But I bet there are subtle details that prevent a simple switch at
> IDT level. Hmm, no low-hanging fruit it seems...


For sure. It's not insurmountable, but fairly hard.

>>
>>> [17] http://thread.gmane.org/gmane.comp.emulators.kvm.devel/26712/
> That thread proposed the complete IST removal. But, given that we still
> have it 7 years later,

Well, it's not as if a crack team of kernel hackers was laboring night 
and day to remove it, but...

>   I suppose that was not very welcome in general.

Simply removing it is impossible, or an NMI happening immediately after 
SYSCALL will hit user-provided %rsp.

> Thanks,
> Jan
>
> PS: For the Jailhouse readers: we don't use IST.
>

You don't have userspace, yes?  Only guests?

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