Re: [Xen-devel] Restoring FPU exception state

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Tian, Kevin
> Sent: Thursday, February 18, 2016 2:31 PM
> 
> >
> > > If the VCPU is descheduled between these two checks, the contents of
> > > FCS/FDS is lost, Windows will notice and BugCheck.
> > >
> > > When saving a VCPUs FPU state, Xen first uses a REX.W prefixed XSAVE and
> > > notices that FIP/FDP[64:32] is non-zero and assumes are REX.W prefixed
> > > XRSTOR is required to restore the full 64-bits of FIP/FDP.  This clears
> > > FCS/FDS.
> > >
> > > On processors with FPCSDS[2] (bit 13) set in CPUID leaf 0x7, sub-leaf
> > > 0x0, do not save FCS/FDS (they always write zeros) and this problem does
> > > not occur, because FCS/FDS never needs to be restored.
> > >
> > > Does anyone any thoughts of a solution for processors without the FPCSDS
> > > feature?
> >
> > One would assume they have a solution to this problem on Hyper-V,
> > but then again their solution may simply be that they don't use REX
> > prefixes anywhere (i.e. namely also not in context switch code). In
> > which case, though, they'd corrupt state of their non-Windows guests.
> >
> > But to answer your question: I have no idea. The handling of these
> > FPU code/data pointers has been an unloved child from the very
> > beginning of the x86-64 architecture. All I could see us doing would
> > be to add a per-domain control to override the auto-detection in the
> > XSAVE code path.
> >
> 
> Interesting. Let me also have a check internally whether there is other
> architectural alternative.

Looks there is no other good alternative. FPCSDS is right introduced
to address this issue, but then it means only for newer platform.

Thanks
Kevin

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