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

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

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Tuesday, December 11, 2018 1:13 AM
> 
> Ping Kevin / Jun.
> 
> On 16/10/2018 19:54, Andrew Cooper wrote:
> > Hello,
> >
> > I realise this is an old CPU, but I've I've encountered a weird failure
> > on it.
> >
> > Specifically:
> >
> > (XEN) CPU Vendor: Intel, Family 6 (0x6), Model 23 (0x17), Stepping 6
> > (raw 00010676)
> > [root@harpertown ~]# head /proc/cpuinfo
> > processor    : 0
> > vendor_id    : GenuineIntel
> > cpu family    : 6
> > model        : 23
> > model name    : Intel(R) Xeon(R) CPU           E5420  @ 2.50GHz
> > stepping    : 6
> > microcode    : 0x60f
> > cpu MHz        : 2493.756
> > cache size    : 6144 KB
> > physical id    : 0
> >
> > In Xen, we use an MSR load list to update EFER on vmentry/exit, when
> > hardware doesn't support the EFER field in the VMCB itself.  This is a
> > change I made in 4.11 to fix a bug with NX handling on context switching.

can you point to the commit number of NX fix?

> >
> > 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. 

btw did you try upgrading to a newer microcode? 

Thanks
Kevin
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