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

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 02/03/16 05:38, Jan Beulich wrote:
> >>> On 03.02.16 at 13:22, <haozhong.zhang@intel.com> wrote:
> > On 02/03/16 02:18, Jan Beulich wrote:
> >> >>> On 03.02.16 at 09:28, <haozhong.zhang@intel.com> 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.
>

Oh yes! We can drop pcommit from domains not having access to host
NVDIMM, just like vmx_wbinvd_intercept() dropping wbinvd from domains
not accessing host iomem and ioport.

> > Any functions suggested, vmx_wbinvd_intercept()?
> 
> A good example, yes.
> 
> Jan
> 

Thanks,
Haozhong