linux-kernel - Re: [PATCH 5/7] x86/mmu: Allocate/free PASID

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Date:   Tue, 28 Apr 2020 15:13:03 -0700
From:   "Jacob Pan (Jun)" <jacob.jun.pan@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Mehta, Sohil" <sohil.mehta@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        jacob.jun.pan@...el.com
Subject: Re: [PATCH 5/7] x86/mmu: Allocate/free PASID

On Tue, 28 Apr 2020 13:59:43 -0700
"Luck, Tony" <tony.luck@...el.com> wrote:

> >> So the driver needs to use flush/drain operations to make sure all
> >> the in-flight work has completed before releasing/re-using the
> >> PASID. 
> > Are you suggesting we should let driver also hold a reference of the
> > PASID?  
> 
> The sequence for bare metal is:
> 
> 	process is queuing requests to DSA
> 	process exits (either deliberately, or crashes, or is killed)
> 	kernel does exit processing
> 	DSA driver is called as part of tear down of "mm"
> 		issues drain/flush commands to ensure that all
> 		queued operations on the PASID for this mm have
> 		completed
> 	PASID can be freed
> 
> There's a 1:1 map from "mm" to PASID ... so reference counting seems
> like overkill. Once the kernel is in the "exit" path, we know that no
> more work can be queued using this PASID.
> 
There are two users of a PASID, mm and device driver(FD). If
either one is not done with the PASID, it cannot be reclaimed. As you
mentioned, it could take a long time for the driver to abort. If the
abort ends *after* mmdrop, we are in trouble.
If driver drops reference after abort/drain PASID is done, then we are
safe.


> -Tony