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Subject: Re: [PATCH v15 06/12] swiotlb: Use is_swiotlb_force_bounce for
 swiotlb data bouncing
To:     Will Deacon <will@kernel.org>, Robin Murphy <robin.murphy@arm.com>
Cc:     Christoph Hellwig <hch@lst.de>, heikki.krogerus@linux.intel.com,
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 <20210706170657.GD20750@willie-the-truck>
From:   Nathan Chancellor <nathan@kernel.org>
Message-ID: <e1c026c6-22c7-8979-4941-de9cfab3863a@kernel.org>
Date:   Tue, 6 Jul 2021 12:14:16 -0700
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Hi Will and Robin,

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.

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,
Nathan