linux-kernel - Re: [PATCH] deal with interrupt shadow state for emulated instruction

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Message-ID: <49E4B654.6060101@redhat.com>
Date:	Tue, 14 Apr 2009 19:14:12 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] deal with interrupt shadow state for emulated instruction

H. Peter Anvin wrote:
> Avi Kivity wrote:
>   
>> The comment about repeating 'mov ss' in the manual has that wonderful
>> word in it, May.  That means we're perfectly allowed to ignore it and
>> just set the flag unconditionally.
>>
>>     
>
> Realistically, though, this should only be done for a limited number of
> sequential instructions.
>
>   

Why?  Do you see a guest filling all of memory with 'mov ss' and 
expecting to break out of it via an interrupt?

>> I doubt we'll ever see a repeated 'mov ss', once is more than enough.
>>     
>
> True enough, except maliciously.
>   

Why do we care?  The guest can only harm itself, and if it wants to 
disable interrupts, it would be a lot easier for it to run a plain 'cli'.

I guess it would be a problem if we emulated 'mov ss' for ordinary 
userspace or vm86 mode, but we don't.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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