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

[Mikulas Patocka]

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.

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
>