RE: [Xen-devel] [PATCH][SVM] remove FFXSR CPUID bit for AMD-V HVM guests

["Woller, Thomas" <thomas.woller@xxxxxxx>]

> >>> "Woller, Thomas" <thomas.woller@xxxxxxx> 01.02.07 00:06 >>>
> >Remove visibility of the FFXSR CPUID bit to an HVM guest.
> >This patch allows HVM Windows x64 to install/boot on AMD-V platforms.
> 
> Would you mind explaining why this is necessary? (Similarly I 
> have a hard time understanding why 3dnow can't be allowed for 
> hvm guests.)
There is no known issue with FFSRX at this time, an alternative (that
works) is to modify the code in long_mode_do_msr_write() to not gp fault
on FFSRX bit being set (this was the original failure).  

        if ( msr_content & ~(EFER_FFSRX | EFER_LME | EFER_LMA | EFER_NX
| EFER_SCE) )
        {
            gdprintk(XENLOG_WARNING, "Trying to set reserved bit in "
                     "EFER: %"PRIx64"\n", msr_content);
            goto gp_fault;
        }

The above code also allows windows to continue installation and function
seemingly properly.

So, to answer your Q, there is not a particular known failure case for
3DNow! Or FFSRX disablement - the issue would be that there has been no
directed testing effort concerning 3DNow and FFSRX usages in the guest
to determine if would be any issues per se.  

tom



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