Re: [Xen-devel] [RFC Design Doc] Add vNVDIMM support for Xen

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.02.16 at 13:22, <haozhong.zhang@xxxxxxxxx> wrote:
> On 02/03/16 02:18, Jan Beulich wrote:
>> >>> On 03.02.16 at 09:28, <haozhong.zhang@xxxxxxxxx> wrote:
>> > On 02/02/16 14:15, Konrad Rzeszutek Wilk wrote:
>> >> > 3.1 Guest clwb/clflushopt/pcommit Enabling
>> >> > 
>> >> >  The instruction enabling is simple and we do the same work as in 
> KVM/QEMU.
>> >> >  - All three instructions are exposed to guest via guest cpuid.
>> >> >  - L1 guest pcommit is never intercepted by Xen.
>> >> 
>> >> I wish there was some watermarks like the PLE has.
>> >> 
>> >> My fear is that an unfriendly guest can issue sfence all day long
>> >> flushing out other guests MMC queue (the writes followed by pcommits).
>> >> Which means that an guest may have degraded performance as their
>> >> memory writes are being flushed out immediately as if they were
>> >> being written to UC instead of WB memory. 
>> > 
>> > pcommit takes no parameter and it seems hard to solve this problem
>> > from hardware for now. And the current VMX does not provide mechanism
>> > to limit the commit rate of pcommit like PLE for pause.
>> > 
>> >> In other words - the NVDIMM resource does not provide any resource
>> >> isolation. However this may not be any different than what we had
>> >> nowadays with CPU caches.
>> >>
>> > 
>> > Does Xen have any mechanism to isolate multiple guests' operations on
>> > CPU caches?
>> 
>> No. All it does is disallow wbinvd for guests not controlling any
>> actual hardware. Perhaps pcommit should at least be limited in
>> a similar way?
>>
> 
> But pcommit is a must that makes writes be persistent on pmem. I'll
> look at how guest wbinvd is limited in Xen.

But we could intercept it on guests _not_ supposed to use it, in order
to simply drop in on the floor.

> Any functions suggested, vmx_wbinvd_intercept()?

A good example, yes.

Jan


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