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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Message-ID: <YmavoKkVu+hd+x0M@myrica>
Date:   Mon, 25 Apr 2022 15:26:40 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Joerg Roedel <joro@...tes.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>, will@...nel.org,
        robin.murphy@....com, zhangfei.gao@...aro.org
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

On Mon, Apr 25, 2022 at 07:18:36AM -0700, Dave Hansen wrote:
> On 4/25/22 06:53, Jean-Philippe Brucker wrote:
> > On Sat, Apr 23, 2022 at 07:13:39PM +0800, zhangfei.gao@...mail.com wrote:
> >>>> On 5.17
> >>>> fops_release is called automatically, as well as iommu_sva_unbind_device.
> >>>> On 5.18-rc1.
> >>>> fops_release is not called, have to manually call close(fd)
> >>> Right that's weird
> >> Looks it is caused by the fix patch, via mmget, which may add refcount of
> >> fd.
> > Yes indirectly I think: when the process mmaps the queue, mmap_region()
> > takes a reference to the uacce fd. That reference is released either by
> > explicit close() or munmap(), or by exit_mmap() (which is triggered by
> > mmput()). Since there is an mm->fd dependency, we cannot add a fd->mm
> > dependency, so no mmget()/mmput() in bind()/unbind().
> > 
> > I guess we should go back to refcounted PASIDs instead, to avoid freeing
> > them until unbind().
> 
> Yeah, this is a bit gnarly for -rc4.  Let's just make sure there's
> nothing else simple we can do.
> 
> How does the IOMMU hardware know that all activity to a given PASID is
> finished?  That activity should, today, be independent of an mm or a
> fd's lifetime.

In the case of uacce, it's tied to the fd lifetime: opening an accelerator
queue calls iommu_sva_bind_device(), which sets up the PASID context in
the IOMMU. Closing the queue calls iommu_sva_unbind_device() which
destroys the PASID context (after the device driver stopped all DMA for
this PASID).

Thanks,
Jean