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

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Date:   Fri, 28 Aug 2020 16:55:25 -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

On Fri, 2020-08-28 at 12:27 +1000, Alexey Kardashevskiy wrote:
> 
> On 28/08/2020 01:32, Leonardo Bras wrote:
> > 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.
> 
> The underlying hardware is PHB3/4 about which the IODA2 Version 2.4
> 6Apr2012.pdf spec says:
> 
> "The number of most significant RPN bits implemented in the TCE is
> dependent on the max size of System Memory to be supported by the platform".
> 
> IODA3 is the same on this matter.
> 
> This is MAX_PHYSMEM_BITS and PHB itself does not have any other limits
> on top of that. So the only real limit comes from MAX_PHYSMEM_BITS and
> where TCE_RPN_BITS comes from exactly - I have no idea.

Well, I created this TCE_RPN_BITS = 52 because the previous mask was a
hardcoded 40-bit mask (0xfffffffffful), for hard-coded 12-bit (4k)
pagesize, and on PAPR+/LoPAR also defines TCE as having bits 0-51
described as RPN, as described before.

IODA3 Revision 3.0_prd1 (OpenPowerFoundation), Figure 3.4 and 3.5.
shows system memory mapping into a TCE, and the TCE also has bits 0-51
for the RPN (52 bits). "Table 3.6. TCE Definition" also shows it.

In fact, by the looks of those figures, the RPN_MASK should always be a
52-bit mask, and RPN = (page >> tceshift) & RPN_MASK.

Maybe that's it?

> 
> 
> > > 
> > > > +#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;
> 
> A mask for a page number (but not the address!) hurts my brain, masks
> are good against addresses but numbers should already have all bits
> adjusted imho, may be it is just me :-/
> 
> 
> > 	
> > 	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!
> >  
> > 
> >