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

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Date:   Tue, 28 Apr 2020 19:07:25 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>
CC:     "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>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>
Subject: RE: [PATCH 5/7] x86/mmu: Allocate/free PASID

> 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.

-Tony