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Re: [Qemu-devel] [PATCH 3/3] target-i386: get CPL from SS.DPL

From: Kevin O'Connor
Subject: Re: [Qemu-devel] [PATCH 3/3] target-i386: get CPL from SS.DPL
Date: Wed, 21 May 2014 10:05:38 -0400
User-agent: Mutt/1.5.23 (2014-03-12)

On Wed, May 21, 2014 at 01:13:21PM +0200, Paolo Bonzini wrote:
> Il 20/05/2014 23:54, Kevin O'Connor ha scritto:
> >On Fri, May 16, 2014 at 09:59:25PM +0200, Paolo Bonzini wrote:
> >>CS.RPL is not equal to the CPL in the few instructions between
> >>setting CR0.PE and reloading CS.  We get this right in the common
> >>case, because writes to CR0 do not modify the CPL, but it would
> >>not be enough if an SMI comes exactly during that brief period.
> >>Were this to happen, the RSM instruction would erroneously set
> >>CPL to the low two bits of the real-mode selector; and if they are
> >>not 00, the next instruction fetch cannot access the code segment
> >>and causes a triple fault.
> >>
> >>However, SS.DPL *is* always equal to the CPL.  In real processors
> >>(AMD only) there is a weird case of SYSRET setting SS.DPL=SS.RPL
> >>from the STAR register while forcing CPL=3, but we do not emulate
> >>that.
> >
> >I was in the process of testing something else, when I encountered a
> >problem with an old MSDOS 6.22 floppy I had.  I tracked it down to an
> >error in one of the commits I did in this series (I sent a fix in a
> >separate email for it).
> >
> >Unfortunately, after I fixed the problem in my patch, your patch above
> >breaks it again.  I think it's another VM86 thing.
> >
> >Steps to reproduce:
> >
> >1 - grab the DOS 6.22 floppy from: http://bootdisk.com/bootdisk.htm
> >
> >2 - boot it up and add emm386.exe to config.sys ("edit config.sys" and
> >add "DEVICE=EMM386.EXE" on the second line of the file).
> >
> >3 - reboot with modified config.sys
> I cannot reproduce this.  I can see the breakage with current master, and I
> can see your patch fixing it.  It keeps working with these changes.  Please
> try branch cpl-queue at git://github.com/bonzini/qemu.git and see if it
> works for you too.

Apologies - somehow your patch 1 got misapplied to my tree.  Testing
with the tree above works fine.


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