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

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Message-ID: <942acb9b23d87594d0b758cc0daf713be836f8e6.camel@gmail.com>
Date:   Thu, 08 Apr 2021 03:20:19 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Alexey Kardashevskiy <aik@...abs.ru>, 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

Hello Michael, thank you for this feedback!
Comments inline:

On Thu, 2021-04-08 at 15:37 +1000, Michael Ellerman wrote:
> 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.

Sure, I will add the information about LoPAR.

> 
> > +#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?

My v1 did not use the define approach, what do you think of that?
http://patchwork.ozlabs.org/project/linuxppc-dev/patch/20210322190943.715368-1-leobras.c@gmail.com/

> 
> > +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

Best regards,
Leonardo Bras