linux-kernel - Re: [PATCH v1 01/10] powerpc/pseries/iommu: Replace hard-coded page shift

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Date:   Thu, 27 Aug 2020 12:32:57 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Joel Stanley <joel@....id.au>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>,
        Brian King <brking@...ux.vnet.ibm.com>,
        Murilo Fossa Vicentini <muvic@...ux.ibm.com>,
        David Dai <zdai@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 01/10] powerpc/pseries/iommu: Replace hard-coded page
 shift

Hello Alexey, thank you for this feedback!

On Sat, 2020-08-22 at 19:33 +1000, Alexey Kardashevskiy wrote:
> > +#define TCE_RPN_BITS		52		/* Bits 0-51 represent RPN on TCE */
> 
> Ditch this one and use MAX_PHYSMEM_BITS instead? I am pretty sure this
> is the actual limit.

I understand this MAX_PHYSMEM_BITS(51) comes from the maximum physical memory addressable in the machine. IIUC, it means we can access physical address up to (1ul << MAX_PHYSMEM_BITS). 

This 52 comes from PAPR "Table 9. TCE Definition" which defines bits
0-51 as the RPN. By looking at code, I understand that it means we may input any address < (1ul << 52) to TCE.

In practice, MAX_PHYSMEM_BITS should be enough as of today, because I suppose we can't ever pass a physical page address over 
(1ul << 51), and TCE accepts up to (1ul << 52).
But if we ever increase MAX_PHYSMEM_BITS, it doesn't necessarily means that TCE_RPN_BITS will also be increased, so I think they are independent values. 

Does it make sense? Please let me know if I am missing something.

> 
> 
> > +#define TCE_RPN_MASK(ps)	((1ul << (TCE_RPN_BITS - (ps))) - 1)
> >  #define TCE_VALID		0x800		/* TCE valid */
> >  #define TCE_ALLIO		0x400		/* TCE valid for all lpars */
> >  #define TCE_PCI_WRITE		0x2		/* write from PCI allowed */
> > diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> > index e4198700ed1a..8fe23b7dff3a 100644
> > --- a/arch/powerpc/platforms/pseries/iommu.c
> > +++ b/arch/powerpc/platforms/pseries/iommu.c
> > @@ -107,6 +107,9 @@ static int tce_build_pSeries(struct iommu_table *tbl, long index,
> >  	u64 proto_tce;
> >  	__be64 *tcep;
> >  	u64 rpn;
> > +	const unsigned long tceshift = tbl->it_page_shift;
> > +	const unsigned long pagesize = IOMMU_PAGE_SIZE(tbl);
> > +	const u64 rpn_mask = TCE_RPN_MASK(tceshift);
> 
> Using IOMMU_PAGE_SIZE macro for the page size and not using
> IOMMU_PAGE_MASK for the mask - this incosistency makes my small brain
> explode :) I understand the history but maaaaan... Oh well, ok.
> 

Yeah, it feels kind of weird after two IOMMU related consts. :)
But sure IOMMU_PAGE_MASK() would not be useful here :)

And this kind of let me thinking:
> > +		rpn = __pa(uaddr) >> tceshift;
> > +		*tcep = cpu_to_be64(proto_tce | (rpn & rpn_mask) << tceshift);
Why not:
	rpn_mask =  TCE_RPN_MASK(tceshift) << tceshift;

	rpn = __pa(uaddr) & rpn_mask;
	*tcep = cpu_to_be64(proto_tce | rpn)

I am usually afraid of changing stuff like this, but I think it's safe.

> Good, otherwise. Thanks,

Thank you for reviewing!