[vbox-dev] Virtualbox don't restore FPU segments with 32-bit guests while using xsave/xrstor

Fri May 13 12:54:12 GMT 2016

Hi Frank,

When we run a FPU instruction on a x86 system, FPU CS and DS are set to the
same value as the CS and DS registers. This is an historical reason where
FPU is an individual chip with its own registers. So, the expected
behaviour of this sample is to run forever on a guest x86.

On a x86_64 system, this is different because it removes this historical
behavior and the FPU segments are always set to 0. So, if you run this
sample on a x86_64 system, it's the normal behaviour to have "segs unset 1".

The problem is that Virtualbox doesn't restore properly FPU CS and DS when
it uses xsave/xrstor, but does it if it uses fxsave/fxrstor. The problem
happens randomly and I think that it's cause by the switch between guest
code execution and host code execution (when Virtualbox save the guest
state, restore the host state and after the host code execution repeated
those operations in reverse order).

My patch fixes that problem by using the same behaviour than for fxsave /
fxrstor to save CS and DS when using xsave / xrstor in that particular case.

Regards,

2016-05-13 14:53 GMT+02:00 quentin buathier <qbuathier at tetrane.com>:

> Hi Frank,
>
> When we run a FPU instruction on a x86 system, FPU CS and DS are set to
> the same value as the CS and DS registers. This is an historical reason
> where FPU is an individual chip with its own registers. So, the expected
> behaviour of this sample is to run forever on a guest x86.
>
> On a x86_64 system, this is different because it removes this historical
> behavior and the FPU segments are always set to 0. So, if you run this
> sample on a x86_64 system, it's the normal behaviour to have "segs unset 1".
>
> The problem is that Virtualbox doesn't restore properly FPU CS and DS when
> it uses xsave/xrstor, but does it if it uses fxsave/fxrstor. The problem
> happens randomly and I think that it's cause by the switch between guest
> code execution and host code execution (when Virtualbox save the guest
> state, restore the host state and after the host code execution repeated
> those operations in reverse order).
>
> My patch fixes that problem by using the same behaviour than for fxsave /
> fxrstor to save CS and DS when using xsave / xrstor in that particular case.
>
> Regards,
>
> 2016-05-13 9:14 GMT+02:00 Frank Mehnert <frank.mehnert at oracle.com>:
>
>> Hi Quentin,
>>
>> what is the expected behaviour of this sample? Should it run forever?
>> Running this sample in a 32-bit guests stops with "segs unset" after
>> a short time. After applying your patch and running the example in the
>> guest, it runs forever.
>>
>> But: If I run this sample on the host (Linux 4.5.4), it will always
>> stop with "segs unset 1" after the first turn.
>>
>> Kind regards,
>>
>> Frank
>>
>> On Thursday 12 May 2016 14:47:01 quentin buathier wrote:
>> > This is a sample in C++ which reproduce the problem randomly (1 ~ 2
>> > seconds).
>> > On the same host / guest / cpu that my previous mail.
>> >
>> > 2016-05-12 12:20 GMT+02:00 quentin buathier <qbuathier at tetrane.com>:
>> > > Hi Michal,
>> > >
>> > > I can't now give a way to reproduce the bug but I'll send an
>> executable if
>> > > I manage to reproduce the problem on something minimalist.
>> > >
>> > > But I can give you the context of the problem:
>> > >  Host OS: Debian jessie 64-bits
>> > >  Guest OS: Debian jessie 32-bits
>> > >  Processor: i7-2600 (and all i7 tested)
>> > >
>> > > PS: Sorry for the previous mail that was accidently sent
>> > >
>> > > Regards,
>> > >
>> > > 2016-05-12 12:18 GMT+02:00 quentin buathier <qbuathier at tetrane.com>:
>> > >> Hi Michal,
>> > >>
>> > >> I can't now give a way to reproduce the bug. I'll send an executable
>> if I
>> > >> manage to reproduce the problem on something minimalist.
>> > >>
>> > >> But I can give you the context of the problem:
>> > >>  Host OS: Debian jessie 64-bits
>> > >>
>> > >> 2016-05-12 11:52 GMT+02:00 Michal Necasek <michal.necasek at oracle.com
>> >:
>> > >>>    Hi Quentin,
>> > >>>
>> > >>>  Thank you for the patch!
>> > >>>
>> > >>>  Unfortunately (?) I can't reproduce the problem that was originally
>> > >>>
>> > >>> fixed. Could you please provide a bit more information? What's the
>> host
>> > >>> OS,
>> > >>> guest OS, host CPU type? How to reproduce the problem?
>> > >>>
>> > >>>     Regards,
>> > >>>
>> > >>>       Michal
>> > >>>
>> > >>> On 5/12/2016 11:26 AM, quentin buathier wrote:
>> > >>>> Hi,
>> > >>>>
>> > >>>> As I understand it, there used to be a problem with restoring the
>> FPU
>> > >>>> segments in case of a 64-bit hosts with a 32-bit guest. This issue
>> has
>> > >>>> been fixed by using the macros "SAVE_32_OR_64_FPU" and
>> > >>>> "RESTORE_32_OR_64_FPU" in "src/VBox/VMM/VMMR0/CPUMR0A.asm" (when
>> > >>>> Virtualbox was using fxsave and fxrstor to save and restore the FPU
>> > >>>> context).
>> > >>>>
>> > >>>> But along with the recent support of xsave / xrstor, the bug was
>> > >>>> reintroduced: if the CPU supports xsave/xrstor, Virtualbox uses
>> these
>> > >>>> instructions and the guest's FPU segments are not restored
>> properly.
>> > >>>>
>> > >>>> Please find attached a possible patch to fix this issue (MIT
>> licence).
>> > >>>>
>> > >>>> Regards,
>> > >>>>
>> > >>>>
>> > >>>> _______________________________________________
>> > >>>> vbox-dev mailing list
>> > >>>> vbox-dev at virtualbox.org
>> > >>>> https://www.virtualbox.org/mailman/listinfo/vbox-dev
>> > >>>
>> > >>> _______________________________________________
>> > >>> vbox-dev mailing list
>> > >>> vbox-dev at virtualbox.org
>> > >>> https://www.virtualbox.org/mailman/listinfo/vbox-dev
>>
>> --
>> Dr.-Ing. Frank Mehnert | Software Development Director, VirtualBox
>> ORACLE Deutschland B.V. & Co. KG | Werkstr. 24 | 71384 Weinstadt, Germany
>>
>> ORACLE Deutschland B.V. & Co. KG
>> Hauptverwaltung: Riesstraße 25, D-80992 München
>> Registergericht: Amtsgericht München, HRA 95603
>>
>> Komplementärin: ORACLE Deutschland Verwaltung B.V.
>> Hertogswetering 163/167, 3543 AS Utrecht, Niederlande
>> Handelsregister der Handelskammer Midden-Niederlande, Nr. 30143697
>> Geschäftsführer: Alexander van der Ven, Jan Schultheiss, Val Maher
>> _______________________________________________
>> vbox-dev mailing list
>> vbox-dev at virtualbox.org
>> https://www.virtualbox.org/mailman/listinfo/vbox-dev
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.virtualbox.org/pipermail/vbox-dev/attachments/20160513/757fd937/attachment.html>