[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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



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)

~Andrew

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.