From: gengdongjiu <gengdongjiu@huawei.com>
To: Marc Zyngier <marc.zyngier@arm.com>, James Morse <james.morse@arm.com>
CC: "christoffer.dall@linaro.org" <christoffer.dall@linaro.org>,
        "vladimir.murzin@arm.com" <vladimir.murzin@arm.com>,
        "rkrcmar@redhat.com" <rkrcmar@redhat.com>,
        "catalin.marinas@arm.com" <catalin.marinas@arm.com>,
        "shankerd@codeaurora.org" <shankerd@codeaurora.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Zhanghaibin (Euler)" <zhanghaibin7@huawei.com>,
        Huangshaoyu <huangshaoyu@huawei.com>
Subject: Re: [PATCH] arm64: KVM: VHE: reset PSTATE.UAO when switch to host
Thread-Topic: [PATCH] arm64: KVM: VHE: reset PSTATE.UAO when switch to host
Thread-Index: AdMn53+cw186Ln3mSg+sX1bkFCBFew==
Date: Thu, 7 Sep 2017 15:03:49 +0000
Message-ID: <0184EA26B2509940AA629AE1405DD7F2015EF473@DGGEMA503-MBX.china.huawei.com>
Accept-Language: zh-CN, en-US
Content-Language: zh-CN
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 8BIT
MIME-Version: 1.0
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2478
Lines: 50

> On 07/09/17 12:49, gengdongjiu wrote:
> >
> >
> > On 2017/9/7 18:13, Marc Zyngier wrote:
> >> On 07/09/17 11:05, gengdongjiu wrote:
> >>> Hi James,
> >>>
> >>> On 2017/9/7 17:20, James Morse wrote:
> >>>> Hi Dongjiu Geng,
> >>>>
> >>>> On 07/09/17 06:54, Dongjiu Geng wrote:
> >>>>> In VHE mode, host kernel runs in the EL2 and can enable 'User
> >>>>> Access Override' when fs==KERNEL_DS so that it can access kernel
> >>>>> memory. However, PSTATE.UAO is set to 0 on an exception taken from
> >>>>> EL1 to EL2. Thus when VHE is used and exception taken from a guest
> >>>>> UAO will be disabled and host will use the incorrect PSTATE.UAO.
> >>>>> So check and reset the PSTATE.UAO when switching to host.
> >>>>
> >>>> This would only be a problem if KVM were calling into world-switch
> >>>> with fs==KERNEL_DS. I can't see where this happens.
> >>>  Not only KVM, may also kernel sets the fs == KERNEL_DS before
> >>> calling into world-switch
> >>
> >> How? Please describe the exact sequence of event that lead to this
> >> situation with the current code base.
> >
> > Hi Marc,
> >
> >    Different tasks have different fs, such as USER_DS or KERNEL_DS. In
> > the context switch, it will restore the task's fs. Thus, that depends
> > on task itself, as shown below code. UAO is different with PAN, PAN will be always enabled if hardware CPU supports PAN feature, but
> UAO is dynamical change.
> 
> You haven't answered my question: There is exactly one point where we enter the world-switch. Show me that, at this point, PSTATE.UAO
> *before* the call is different from PSTATE.UAO after the call. Give me the exact sequence of event that leads to this situation. Show me a
> stack trace.

Hi Marc,

  If using current mainline KVM code + Qemu and modify nothing, may not exist broken issue,  because the Qemu progress FS should be USER_DS. But if I make a modification for user space( Qemu or KVM tools) to change its FS property to KERNEL_DS or use third party application with KERNEL_DS FS to run the guest, it will have problem(the KERNEL_DS is cleared). If you think my case is reasonable and should support, I can show you the calling stack trace. If you think, my case is not reasonable and KVM should not support the application with KERNEL_DS fs to run guest. You can ignore this patch, thanks.


> 
> Until you do this, I will ignore any further comment coming from you on this subject.
> 
> Thanks,
> 
> 	M.
> --
> Jazz is not dead. It just smells funny...