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

[Tim Deegan <tim@xxxxxxx>]

At 14:53 +0100 on 17 Aug (1439823232), Ben Catterall wrote:
> On 12/08/15 14:33, Andrew Cooper wrote:
> > On 12/08/15 14:29, Andrew Cooper wrote:
> >> On 11/08/15 19:29, Boris Ostrovsky wrote:
> >>> Would switching TR only when we know that we need to enter this
> >>> deprivileged mode help?
> >> This is an absolute must.  It is not safe to use syscall/sysexit without
> >> IST in place for NMIs and MCEs.
> >>
> >>> Assuming that it is less expensive than copying the stack.
> >> I was referring to the stack overflow issue, and whether it might be
> >> sensible to pro-actively which TR.
> >
> > Ahem! s/which/switch/
> >
> > ~Andrew
> >
> 
> So, have we arrived at a decision for this? Thanks!

Seems to have stalled a bit.  OK, I propose that:
 - we use TR/IST to make Xen take interrupts/exceptions at a different SP;
 - we make that SP be an extension of the main stack, so that things
   like current() Just Work[tm];
 - we set this up and tear it down when we enter/leave depriv mode.
 - someone ought to look at the case where IST handlers copy
   themselves to the main stack, and see if we need to adjust that too.

Any other proposals?

I think we can leave the question of TR switching on VMEXIT as a
separate issue.

Cheers,

Tim.

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