linux-kernel - Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

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Date:   Thu, 18 Mar 2021 23:35:00 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>
Cc:     Jonathan Corbet <corbet@....net>, opendmb@...il.com,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "open list:SWIOTLB SUBSYSTEM" <iommu@...ts.linux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

On 2021-03-18 21:31, Florian Fainelli wrote:
> 
> 
> On 3/18/2021 12:53 PM, Robin Murphy wrote:
>> On 2021-03-18 19:43, Florian Fainelli wrote:
>>>
>>>
>>> On 3/18/2021 12:34 PM, Robin Murphy wrote:
>>>> On 2021-03-18 19:22, Florian Fainelli wrote:
>>>>>
>>>>>
>>>>> On 3/18/2021 12:18 PM, Florian Fainelli wrote:
>>>>>> It may be useful to disable the SWIOTLB completely for testing or
>>>>>> when a
>>>>>> platform is known not to have any DRAM addressing limitations what so
>>>>>> ever.
>>>>
>>>> Isn't that what "swiotlb=noforce" is for? If you're confident that we've
>>>> really ironed out *all* the awkward corners that used to blow up if
>>>> various internal bits were left uninitialised, then it would make sense
>>>> to just tweak the implementation of what we already have.
>>>
>>> swiotlb=noforce does prevent dma_direct_map_page() from resorting to the
>>> swiotlb, however what I am also after is reclaiming these 64MB of
>>> default SWIOTLB bounce buffering memory because my systems run with
>>> large amounts of reserved memory into ZONE_MOVABLE and everything in
>>> ZONE_NORMAL is precious at that point.
>>
>> It also forces io_tlb_nslabs to the minimum, so it should be claiming
>> considerably less than 64MB. IIRC the original proposal *did* skip
>> initialisation completely, but that turned up the aforementioned issues.
> 
> AFAICT in that case we will have iotlb_n_slabs will set to 1, which will
> still make us allocate io_tlb_n_slabs << IO_TLB_SHIFT bytes in
> swiotlb_init(), which still gives us 64MB.

Eh? When did 2KB become 64MB? IO_TLB_SHIFT is 11, so that's at most one 
page in anyone's money...

>>>> I wouldn't necessarily disagree with adding "off" as an additional alias
>>>> for "noforce", though, since it does come across as a bit wacky for
>>>> general use.
>>>>
>>>>>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>>>>>
>>>>> Christoph, in addition to this change, how would you feel if we
>>>>> qualified the swiotlb_init() in arch/arm/mm/init.c with a:
>>>>>
>>>>>
>>>>> if (memblock_end_of_DRAM() >= SZ_4G)
>>>>>       swiotlb_init(1)
>>>>
>>>> Modulo "swiotlb=force", of course ;)
>>>
>>> Indeed, we would need to handle that case as well. Does it sound
>>> reasonable to do that to you as well?
>>
>> I wouldn't like it done to me personally, but for arm64, observe what
>> mem_init() in arch/arm64/mm/init.c already does.

In fact I should have looked more closely at that myself - checking 
debugfs on my 4GB arm64 board actually shows io_tlb_nslabs = 0, and 
indeed we are bypassing initialisation completely and (ab)using 
SWIOTLB_NO_FORCE to cover it up, so I guess it probably *is* safe now 
for the noforce option to do the same for itself and save even that one 
page.

Robin.