Re: [Xen-devel] MSR load lists on Harpertown

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

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Monday, December 17, 2018 10:21 PM
> 
> On 17/12/2018 13:09, Andrew Cooper wrote:
> > On 17/12/2018 02:39, Tian, Kevin wrote:
> >>>>> After some investigation, it turns out that after vmentry, while the
> >>>>> load list has the value 0xd01 (NXE, LMA, LME, SCE), the value loaded
> >>>>> into hardware is 0xd00 (NXE, LMA, LME).
> >>>>>
> >>>>> I.e. when an MSR load list is used for EFER, we resume the guest with
> >>>>> SCE cleared.  This is rather terminal for 64bit guests, as
> >>>>> syscall/sysret instructions take a #UD fault.
> >>>>>
> >>>>> I can't see anything relevant in the Specification Update for this
> >>>>> processor.
> >>>>>
> >>>>> I've confirmed that by not using a load list, the current value in EFER
> >>>>> is preserved once the vmentry is complete, and by disabling the EFER
> >>>>> intercept, I can re-set SCE in non-root context and have syscall/sysret
> >>>>> work correctly.
> >>>>>
> >>>>> However, given this behaviour, I can't think of any way to context
> >>>>> switch NX properly, and leave 64bit guests in a working state.
> >>>>>
> >>>>> Do you have any suggestions?
> >>>>>
> >>> I'm checking internally whether it's a known issue.
> >> from feedbacks that I collected so far, no one is aware of this issue.
> >>
> >>> btw did you try upgrading to a newer microcode?
> >>>
> >> while I'm approaching more channels, does it work by directly
> >> WRMSR to EFER just before VMENTRY for above special case (
> >> thus remove EFER from MSR load/save list), if ucode update
> >> also fails? there is just a small window where NX might be wrong
> >> setting for Xen, but it might be OK for that carefully-baked code
> >> snippet?
> > Apologies for the delay.  I was travelling last week.
> >
> > We cannot load the full guest's EFER value in Xen context.  If the guest
> > has NX disabled, the next stack access in Xen will fault because the NX
> > bit becomes reserved when EFER.NXE is clear.
> >
> > As for the more general case of loading the guests EFER value (ignoring
> > NXE), we already know that works, because it is how Xen functioned for a
> > decade.
> >
> > I see that the latest production microcode on otcshare is slightly newer
> > than exists in the microcode_ctl package.  I'll give it a spin.
> 
> The behaviour is still the same, even with the latest microcode:
> 
> (XEN) microcode: CPU0 updated from revision 0x60f to 0x612, date =
> 2015-08-02
> 

Just confirmed that there is no erratum which could explain above 
behavior.

I see several options here:

a) load guest EFER in Xen context, just before vmentry. make sure no stack 
access between load and VM enter. but it sounds messy to add such sku
specific workaround in that common path (especially just for an old one);

b) leave guest following Xen NXE setting on Harpertown. doing so bears
with limitation as what fd32dcfe tries to fix, but it's better than syscall
error in 64bit guest. also not clean since some ad-hoc logic that is removed
by fd32dcfe may have to be added back;

c) leave above issue unfixed. Harpertown is pretty old...

d) more debug whether above is caused by other software bug

Thanks
Kevin
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