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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 21/05/2014 16:05, Kevin O'Connor ha scritto:
> 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.

Should I take this as a Tested-by? :)

Paolo