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

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Date:   Thu, 8 Jul 2021 17:44:19 +0100
From:   Will Deacon <will@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        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>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        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,
        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 12:14:16PM -0700, Nathan Chancellor wrote:
> On 7/6/2021 10:06 AM, Will Deacon wrote:
> > On Tue, Jul 06, 2021 at 04:39:11PM +0100, Robin Murphy wrote:
> > > On 2021-07-06 15:05, 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.
> > > 
> > > Ah yes, that had slipped my mind, and it's a fair point indeed. Since we're
> > > not concerned with a minimal fix for backports anyway I'm more than happy to
> > > focus on Will's approach. Another thing is that that looks to take us a
> > > quiet step closer to the possibility of dynamically resizing a SWIOTLB pool,
> > > which is something that some of the hypervisor protection schemes looking to
> > > build on top of this series may want to explore at some point.
> > 
> > Ok, I'll split that nasty diff I posted up into a reviewable series and we
> > can take it from there.
> 
> For what it's worth, I attempted to boot Will's diff on top of Konrad's
> devel/for-linus-5.14 and it did not work; in fact, I got no output on my
> monitor period, even with earlyprintk=, and I do not think this machine has
> a serial console.

Looking back at the diff, I completely messed up swiotlb_exit() by mixing up
physical and virtual addresses.

> Robin's fix does work, it survived ten reboots with no issues getting to X
> and I do not see the KASAN and slub debug messages anymore but I understand
> that this is not the preferred solution it seems (although Konrad did want
> to know if it works).
> 
> I am happy to test any further patches or follow ups as needed, just keep me
> on CC.

Cheers. Since this isn't 5.14 material any more, I'll CC you on a series
next week.

Will