Message-ID: <49DCCCCB.9000201@zytor.com>
Date: Wed, 08 Apr 2009 09:11:55 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Glauber Costa <glommer@redhat.com>
CC: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, avi@redhat.com
Subject: Re: [PATCH] do not keep interrupt window closed by sti in real mode
References: <1239161017-7398-1-git-send-email-glommer@redhat.com> <49DC24C2.7000608@zytor.com> <20090408145545.GA25323@poweredge.glommer>
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Glauber Costa wrote:
> 
> I get the idea from the pseudocode in sti description.
> It says:
> IF PE = 0 (* Executing in real-address mode *)
>     THEN
>         IF <- 1; (* Set Interrupt Flag *)
>     ELSE (* Executing in protected mode or virtual-8086 mode *)
> 
> There is no mention to any other activity besides setting the if flag.

But the same is true for the protected mode side of the instruction 
description!

> Also, sti is used extensively in many places like the linux kernel for the
> guest, and it works just fine in kvm. So I was led to believe that real mode
> in fact behaving differently.

The difference is that at least under current Intel VT, VT only handles 
protected mode -- the real mode runs purely in the interpreter.

> I'll take a look at avi's suggestion.

	-hpa
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