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

Michal Necasek michal.necasek at oracle.com
Thu May 12 09:52:15 UTC 2016

    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?


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,
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