Re: trap/interrupt gate for hypercall

[Keir Fraser <keir@xensource.com>]

On 15/5/07 16:18, "Tian, Kevin" <kevin.tian@intel.com> wrote:

>> Everything's an interrupt gate on 32-bit Xen, so that we can safely
>> (atomically) save away guest segment register state. NMI is the only real
>> pain, and I suppose MCE too. Coincidentally Jan just emailed about this
>> very point.
> 
> So why is atomic save required for hypercall here? NMI can break
> atomic save here for both types, as you said. Normally external
> interrupt handler doesn't need communicate with interrupted
> context. So even when an external interrupt comes in the middle
> of the guest context save for hypercall (if by trap gate), that
> interrupted save process can still continue since interrupt handler
> itself will impose atomic context save/restore on top of interrupted
> context.

Interrupt handlers save and restore segment registers. We could fault on a
reload of a segment register and lose the original segment register value.

 -- Keir