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

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Date:   Wed, 8 Nov 2017 07:24:29 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Haozhong Zhang <haozhong.zhang@...el.com>
cc:     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>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 0/2] KVM: MMU: fix kvm_is_mmio_pfn()



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