linux-kernel - Re: [PATCH v2 0/2] KVM: MMU: fix kvm_is_mmio

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Date:   Fri, 3 Nov 2017 08:26:57 +0800
From:   Haozhong Zhang <haozhong.zhang@...el.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Olif Chapman <olif.chapman@...cle.com>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        ivan.d.cuevas.escareno@...el.com, karthik.kumar@...el.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 0/2] KVM: MMU: fix kvm_is_mmio_pfn()

On 11/02/17 13:37 -0400, Konrad Rzeszutek Wilk wrote:
> On Tue, Oct 31, 2017 at 07:48:38PM +0800, 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
> > 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.
> 
> Is there a specific workload you used to detect this?
>

Creating files on NVDIMM in VM is must slower than the same operation
on the baremetal. As the wrong EPT memory type (UC vs. WB) is used,
every guest access to NVDIMM would be slower than baremetal.

Haozhong

> Thanks!
> > 
> > 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
> >