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

On 17/08/15 08:07, Tim Deegan wrote:
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/


So, have we arrived at a decision for this? Thanks!

Apologies for the delay - I am currently at the Xen Developer Summit.

Seems to have stalled a bit.  OK, I propose that:
  - we use TR/IST to make Xen take interrupts/exceptions at a different SP;

Xen re-enables interrupts in most interrupt handlers, which means that they must not have an IST set. If an IST was set, a second interrupt would clobber the frame of the first.

However, just adjusting tss->rsp0 and syscall top-of-stack to the current rsp when entering depriv mode should be sufficient, and will avoid needing to copy the stack.

  - 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.

They will need adjusting, but just disabling the copy entirely should be ok.

Any other proposals?

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

Agreed.  It is orthogonal to this problem.


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