Re: [PATCH v2 0/2] KVM: MMU: fix kvm_is_mmio_pfn()

From: Mikulas Patocka
Date: Wed Nov 08 2017 - 07:24:40 EST




On Wed, 8 Nov 2017, Haozhong Zhang wrote:

> On 11/03/17 10:15 -0400, Mikulas Patocka wrote:
> >
> >
> > On Tue, 31 Oct 2017, Haozhong Zhang wrote:
> >
> > > [I just copy the commit message from patch 2]
> > >
> > > Some reserved pages, such as those from NVDIMM DAX devices, are
> > > not for MMIO, and can be mapped with cached memory type for better
> > > performance. However, the above check misconceives those pages as
> >
> > Note that cached memory type on persistent memory has horrible
> > performance. The clwb instruction on Broadwell is very slow - when you
> > write to persistent memory and use clwb to flush cache, the performance is
> > about 350MB/s.
>
> Wasn't clwb first introduced on Skylake?
>
> Haozhong

You're right - the clwb() function uses clflushopt on Broadwell - that's
what I meant is slow.

Mikulas