linux-kernel - Re: [PATCH v15 06/12] swiotlb: Use is_swiotlb_force

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Date:   Tue, 6 Jul 2021 10:46:07 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Robin Murphy <robin.murphy@....com>, Will Deacon <will@...nel.org>,
        heikki.krogerus@...ux.intel.com, thomas.hellstrom@...ux.intel.com,
        peterz@...radead.org, benh@...nel.crashing.org,
        joonas.lahtinen@...ux.intel.com, dri-devel@...ts.freedesktop.org,
        chris@...is-wilson.co.uk, grant.likely@....com, paulus@...ba.org,
        Frank Rowand <frowand.list@...il.com>, mingo@...nel.org,
        Stefano Stabellini <sstabellini@...nel.org>,
        Saravana Kannan <saravanak@...gle.com>, mpe@...erman.id.au,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        bskeggs@...hat.com, linux-pci@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Thierry Reding <treding@...dia.com>,
        intel-gfx@...ts.freedesktop.org, matthew.auld@...el.com,
        linux-devicetree <devicetree@...r.kernel.org>,
        Jianxiong Gao <jxgao@...gle.com>,
        Daniel Vetter <daniel@...ll.ch>,
        maarten.lankhorst@...ux.intel.com, airlied@...ux.ie,
        Dan Williams <dan.j.williams@...el.com>,
        linuxppc-dev@...ts.ozlabs.org, jani.nikula@...ux.intel.com,
        Nathan Chancellor <nathan@...nel.org>,
        Rob Herring <robh+dt@...nel.org>, rodrigo.vivi@...el.com,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Claire Chang <tientzu@...omium.org>,
        boris.ostrovsky@...cle.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        jgross@...e.com, Nicolas Boichat <drinkcat@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Qian Cai <quic_qiancai@...cinc.com>,
        lkml <linux-kernel@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Jim Quinlan <james.quinlan@...adcom.com>, xypron.glpk@....de,
        Tom Lendacky <thomas.lendacky@....com>, bauerman@...ux.ibm.com
Subject: Re: [PATCH v15 06/12] swiotlb: Use is_swiotlb_force_bounce for
 swiotlb data bouncing

On Tue, Jul 06, 2021 at 04:05:13PM +0200, Christoph Hellwig wrote:
> On Tue, Jul 06, 2021 at 03:01:04PM +0100, Robin Murphy wrote:
> > FWIW I was pondering the question of whether to do something along those 
> > lines or just scrap the default assignment entirely, so since I hadn't got 
> > round to saying that I've gone ahead and hacked up the alternative 
> > (similarly untested) for comparison :)
> >
> > TBH I'm still not sure which one I prefer...
> 
> Claire did implement something like your suggestion originally, but
> I don't really like it as it doesn't scale for adding multiple global
> pools, e.g. for the 64-bit addressable one for the various encrypted
> secure guest schemes.

Couple of things:
 - I am not pushing to Linus the Claire's patchset until we have a
   resolution on this. I hope you all agree that is a sensible way
   forward as much as I hate doing that.

 - I like Robin's fix as it is simplest looking. Would love to see if it
   does fix the problem.

 - Christopher - we can always add multiple pools as the next milestone
   and just focus on this feature getting tested extensively during this
   release.

 - Would it be worth (for future or maybe in another tiny fix) to also add
   a printk in swiotlb when we de-allocate the buffer so when someone looks
   through the `dmesg` it becomes much easier to diagnose issues?