Re: [Xen-devel] [RFC 3/4] HVM x86 deprivileged mode: Code for switching into/out of deprivileged mode

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 08/11/2015 01:19 PM, Andrew Cooper wrote:
> On 11/08/15 18:05, Tim Deegan wrote:
> > > > > * Under this model, PV exception handlers should copy themselves onto
> > > > > the privileged execution stack.
> > > > > * Currently, the IST handlers  copy themselves onto the primary stack if
> > > > > they interrupt guest context.
> > > > > * AMD Task Register on vmexit.  (this old gem)
> > > > Gah, this thing. :
> > > Curious (and I can't seem find this in the manuals): What is this thing?
> > IIRC: AMD processors don't context switch TR on vmexit,
> Correct
>
> > which makes using IST handlers tricky there.
> (That is one way of putting it)
>
> IST handlers cannot be used by Xen if Xen does not switch the task
> register before stgi, or IST exceptions (NMI, MCE and double fault) will
> be taken with guest-supplied stack pointers.
>
> > We'd have to do the TR context switch ourselves, and that would be expensive.
> It is suspected to be expensive, but I have never actually seen any
> numbers one way or another.
>
> > Andrew, am I remembering that right?
> Looks about right.
>
> I have been meaning to investigate this for a while, but never had the time.
>
> Xen opts for disabling interrupt stack tables in the context of AMD HVM
> vcpus, which interacts catastrophically with debug builds using
> MEMORY_GUARD.  MEMORY_GUARD shoots a page out of the primary stack to
> detect stack overflows, but without an IST double fault hander, ends in
> a triple fault rather than a host crash detailing the stack overflow.
>
> KVM unilaterally reloads the host task register on vmexit, and I suspect
> this is probably the way to go, but have not had time to investigate
> whether there is any performance impact from doing so.  Given how little
> of a TSS is actually used in long mode, I wouldn't expect an `ltr` to be
> as expensive as it might have been in legacy modes.
>
> (CC'ing the AMD SVM maintainers to see if they have any information on
> this subject)
I actually didn't even realize that TR is not saved on vmexit ;-/.

Would switching TR only when we know that we need to enter this 
deprivileged mode help? Assuming that it is less expensive than copying 
the stack.
-boris

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