linux-kernel - Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

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Date:   Fri, 15 Apr 2022 02:59:32 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>,
        Joerg Roedel <joro@...tes.org>,
        jean-philippe <jean-philippe@...aro.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

Hi, Dave,

On Tue, Apr 12, 2022 at 07:39:10AM -0700, Dave Hansen wrote:
> On 4/12/22 06:41, Fenghua Yu wrote:
> >> master process quit, mmput ->  mm_pasid_drop->ioasid_free
> >> But this ignore driver's iommu_sva_unbind_device function,
> >> iommu_sva_bind_device and iommu_sva_unbind_device are not pair,  So driver
> >> does not know ioasid is freed.
> >>
> >> Any suggestion?
> > ioasid is per process or per mm. A daemon process shouldn't share the same 
> > ioasid with any other process with even its parent process. Its parent gets
> > an ioasid and frees it on exit. The ioasid is gone and shouldn't be used
> > by its child process.
> > 
> > Each daemon process should call driver -> iommu_sva_bind_device -> ioasid_alloc
> > to get its own ioasid/PASID. On daemon quit, the ioasid is freed.
> > 
> > That means nqnix needs to be changed.
> 
> Fenghua, please step back for a second and look at what you are saying.
>  Your patch caused userspace to break.  Now, you're telling someone that
> they need to go change that userspace to work around something that your
> patch.  How, exactly, are you suggesting that nginx could change to fix
> this?  What, specifically, was it doing with *fork()* that was wrong?
> 
> It sounds to me like you're saying that it's OK to break userspace.

You are right. The patch should not break userspace. I follow your
suggestion to fix the issue by mmget() in binding and mmput() in unbinding.
The RFC patch was sent out in another thread. Please review it.

Thank you very much for your advice.

-Fenghua