linux-kernel - Re: [PATCH v2 2/3] doc: swiotlb: Document SWIOTLB areas parameter

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Message-ID: <20240509081403.28c424d5@meshulam.tesarici.cz>
Date: Thu, 9 May 2024 08:14:03 +0200
From: Petr Tesařík <petr@...arici.cz>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet
 <corbet@....net>, Tianyu Lan <Tianyu.Lan@...rosoft.com>,
 mhklinux@...look.com, robin.murphy@....com, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 2/3] doc: swiotlb: Document SWIOTLB areas parameter

On Tue,  7 May 2024 01:34:59 +0000
"T.J. Mercier" <tjmercier@...gle.com> wrote:

> Commit 20347fca71a3 ("swiotlb: split up the global swiotlb lock") added
> the ability to specify the number of SWIOTLB areas, but boot-options.rst
> was not updated as part of that commit.
> 
> Reported-by: Michael Kelley <mhklinux@...look.com>
> Fixes: 20347fca71a3 ("swiotlb: split up the global swiotlb lock")
> Signed-off-by: T.J. Mercier <tjmercier@...gle.com>
> ---
>  Documentation/arch/x86/x86_64/boot-options.rst | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/arch/x86/x86_64/boot-options.rst b/Documentation/arch/x86/x86_64/boot-options.rst
> index a37139d1752f..18161657b301 100644
> --- a/Documentation/arch/x86/x86_64/boot-options.rst
> +++ b/Documentation/arch/x86/x86_64/boot-options.rst
> @@ -287,9 +287,11 @@ iommu options only relevant to the AMD GART hardware IOMMU:
>  iommu options only relevant to the software bounce buffering (SWIOTLB) IOMMU
>  implementation:
>  
> -    swiotlb=<slots>[,force,noforce]
> +    swiotlb=<slots>[,<areas>,force,noforce]

This is not quite how this parameter is parsed. All the following
specifications are valid:

- swiotlb=512        // only slots
- swiotlb=,4         // only areas
- swiotlb=512,4      // slots and areas
- swiotlb=force      // default size
- swiotlb=512,force  // default areas
- swiotlb=512,4,force // explicitly specify everything

I believe the syntax should be somethig like:

  swiotlb={ | [<slots>][,<areas>],}{ force | noforce }

Petr T

>        <slots>
>          Prereserve that many 2K slots for the software IO bounce buffering.
> +      <areas>
> +        Number of SWIOTLB areas with their own lock. Must be a power of 2.
>        force
>          Force all IO through the software TLB.
>          Hardware IOMMU implementations can use SWIOTLB bounce buffering in