linux-kernel - Re: [PATCH 0/3] KVM: MMU: fix kvm_is_mmio

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Message-ID: <6828fd44-f883-3f90-3e7c-1246300f4c41@gmail.com>
Date:   Tue, 31 Oct 2017 16:12:29 +0800
From:   Xiao Guangrong <guangrong.xiao@...il.com>
To:     Haozhong Zhang <haozhong.zhang@...el.com>, kvm@...r.kernel.org,
        x86@...nel.org
Cc:     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 0/3] KVM: MMU: fix kvm_is_mmio_pfn()



On 10/27/2017 10:25 AM, Haozhong Zhang wrote:
> [I just copy the commit message from patch 3]
> 
> By default, KVM treats a reserved page as for MMIO purpose, and maps
> it to guest with UC memory type.  However, some reserved pages are not
> for MMIO, such as pages of DAX device (e.g., /dev/daxX.Y). Mapping
> them with UC memory type will harm the performance.  In order to
> exclude those cases, we check the host cache mode in addition and only
> treat UC/UC- pages as MMIO.
> 

I am afraid that is not only a performance issue but also a architecture
bug - it could trigger MCE as there is alias memory mapping (a page mapped
as both WB and UC).

It may hurt mdev device as well as the device memory may be both mapped
at host and VM.