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RE: [Xen-devel] trap/interrupt gate for hypercall

To: "Keir Fraser" <keir@xxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] trap/interrupt gate for hypercall
From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date: Tue, 15 May 2007 23:18:57 +0800
Cc: "Liu, Jinsong" <jinsong.liu@xxxxxxxxx>
Delivery-date: Tue, 15 May 2007 08:17:24 -0700
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Thread-topic: [Xen-devel] trap/interrupt gate for hypercall
>From: Keir Fraser [mailto:keir@xxxxxxxxxxxxx]
>Sent: 2007年5月15日 23:02
>On 15/5/07 15:43, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
>> Just a curious question about IDT descriptor type for hypercall. What's
>> the reason to use interrupt-gate type (14) for hypercall (0x82) on 32bit
>> Xen?
>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
> -- Keir

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 


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