linux-kernel - Re: [PATCH] do not keep interrupt window closed by sti in real mode

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Message-ID: <49DCCCCB.9000201@zytor.com>
Date:	Wed, 08 Apr 2009 09:11:55 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com
Subject: Re: [PATCH] do not keep interrupt window closed by sti in real mode

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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