linux-kernel - Re: [PATCH v2 1/1] powerpc/iommu: Enable remaining IOMMU Pagesizes present in LoPAR

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Message-ID: <87im4xe3pk.fsf@mpe.ellerman.id.au>
Date:   Thu, 08 Apr 2021 15:37:59 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Leonardo Bras <leobras.c@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Leonardo Bras <leobras.c@...il.com>, brking@...ux.vnet.ibm.com
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] powerpc/iommu: Enable remaining IOMMU Pagesizes
 present in LoPAR

Leonardo Bras <leobras.c@...il.com> writes:
> According to LoPAR, ibm,query-pe-dma-window output named "IO Page Sizes"
> will let the OS know all possible pagesizes that can be used for creating a
> new DDW.
>
> Currently Linux will only try using 3 of the 8 available options:
> 4K, 64K and 16M. According to LoPAR, Hypervisor may also offer 32M, 64M,
> 128M, 256M and 16G.

Do we know of any hardware & hypervisor combination that will actually
give us bigger pages?

> Enabling bigger pages would be interesting for direct mapping systems
> with a lot of RAM, while using less TCE entries.
>
> Signed-off-by: Leonardo Bras <leobras.c@...il.com>
> ---
>  arch/powerpc/platforms/pseries/iommu.c | 49 ++++++++++++++++++++++----
>  1 file changed, 42 insertions(+), 7 deletions(-)
>
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 9fc5217f0c8e..6cda1c92597d 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -53,6 +53,20 @@ enum {
>  	DDW_EXT_QUERY_OUT_SIZE = 2
>  };

A comment saying where the values come from would be good.

> +#define QUERY_DDW_PGSIZE_4K	0x01
> +#define QUERY_DDW_PGSIZE_64K	0x02
> +#define QUERY_DDW_PGSIZE_16M	0x04
> +#define QUERY_DDW_PGSIZE_32M	0x08
> +#define QUERY_DDW_PGSIZE_64M	0x10
> +#define QUERY_DDW_PGSIZE_128M	0x20
> +#define QUERY_DDW_PGSIZE_256M	0x40
> +#define QUERY_DDW_PGSIZE_16G	0x80

I'm not sure the #defines really gain us much vs just putting the
literal values in the array below?

> +struct iommu_ddw_pagesize {
> +	u32 mask;
> +	int shift;
> +};
> +
>  static struct iommu_table_group *iommu_pseries_alloc_group(int node)
>  {
>  	struct iommu_table_group *table_group;
> @@ -1099,6 +1113,31 @@ static void reset_dma_window(struct pci_dev *dev, struct device_node *par_dn)
>  			 ret);
>  }
>  
> +/* Returns page shift based on "IO Page Sizes" output at ibm,query-pe-dma-window. See LoPAR */
> +static int iommu_get_page_shift(u32 query_page_size)
> +{
> +	const struct iommu_ddw_pagesize ddw_pagesize[] = {
> +		{ QUERY_DDW_PGSIZE_16G,  __builtin_ctz(SZ_16G)  },
> +		{ QUERY_DDW_PGSIZE_256M, __builtin_ctz(SZ_256M) },
> +		{ QUERY_DDW_PGSIZE_128M, __builtin_ctz(SZ_128M) },
> +		{ QUERY_DDW_PGSIZE_64M,  __builtin_ctz(SZ_64M)  },
> +		{ QUERY_DDW_PGSIZE_32M,  __builtin_ctz(SZ_32M)  },
> +		{ QUERY_DDW_PGSIZE_16M,  __builtin_ctz(SZ_16M)  },
> +		{ QUERY_DDW_PGSIZE_64K,  __builtin_ctz(SZ_64K)  },
> +		{ QUERY_DDW_PGSIZE_4K,   __builtin_ctz(SZ_4K)   }
> +	};


cheers