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Re: [Xen-devel] MSR load lists on Harpertown
- To: Jun Nakajima <jun.nakajima@xxxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 10 Dec 2018 17:12:59 +0000
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- Cc: Jan Beulich <JBeulich@xxxxxxxx>, Xen-devel List <xen-devel@xxxxxxxxxxxxx>
- Delivery-date: Mon, 10 Dec 2018 17:13:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
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.
>
> 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?
>
> Thanks,
>
> ~Andrew
>
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