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

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, Keir Fraser <keir@xxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] trap/interrupt gate for hypercall
From: Keir Fraser <keir@xxxxxxxxxxxxx>
Date: Tue, 15 May 2007 16:30:16 +0100
Cc: "Liu, Jinsong" <jinsong.liu@xxxxxxxxx>
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Thread-topic: [Xen-devel] trap/interrupt gate for hypercall
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On 15/5/07 16:18, "Tian, Kevin" <kevin.tian@xxxxxxxxx> 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

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