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

[Haozhong Zhang]

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

> 
> Using write-combining memory type for persistent memory is much faster, it 
> can sustain performance of one 8-byte write per tick.
> 
> Mikulas
> 
> > MMIO.  Because KVM maps MMIO pages with UC memory type, the
> > performance of guest accesses to those pages would be harmed.
> > Therefore, we check the host memory type by lookup_memtype() in
> > addition and only treat UC/UC- pages as MMIO.
> > 
> > Changes in v2:
> >  * Switch to lookup_memtype() to get host memory type.
> >  * Rewrite the comment in KVM MMU patch.
> >  * Remove v1 patch 2, which is not necessary in v2.
> > 
> > Haozhong Zhang (2):
> >   x86/mm: expose lookup_memtype()
> >   KVM: MMU: consider host cache mode in MMIO page check
> > 
> >  arch/x86/include/asm/pat.h |  2 ++
> >  arch/x86/kvm/mmu.c         | 30 +++++++++++++++++++++++++++---
> >  arch/x86/mm/pat.c          |  3 ++-
> >  3 files changed, 31 insertions(+), 4 deletions(-)
> > 
> > -- 
> > 2.14.1
> >