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

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Date:   Tue, 28 Apr 2020 13:42:00 -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 12:07:25 -0700
"Luck, Tony" <tony.luck@...el.com> wrote:

> > If fd release cleans up then how should there be something in
> > flight at the final mmdrop?  
> 
> ENQCMD from the user is only synchronous in that it lets the user
> know their request has been added to a queue (or not).  Execution of
> the request may happen later (if the device is busy working on
> requests for other users).  The request will take some time to
> complete. Someone told me the theoretical worst case once, which I've
> since forgotten, but it can be a long time.
> 
> 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?

> -Tony