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

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,

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.


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