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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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?

>> >  - L1 hypervisor is allowed to intercept L2 guest pcommit.
>> 
>> clwb?
> 
> VMX is not capable to intercept clwb. Any reason to intercept it?

I don't think so - otherwise normal memory writes might also need
intercepting. Bus bandwidth simply is shared (and CLWB operates
on a guest virtual address, so can only cause bus traffic for cache
lines the guest has managed to dirty).

Jan


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