linux-kernel - Re: [PATCH v1 02/10] powerpc/kernel/iommu: Align size for IOMMU_PAGE_SIZE on iommu_*

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Message-ID: <c67c66e466ad27d15aa2b970c48d2336d95b2971.camel@gmail.com>
Date:   Thu, 27 Aug 2020 13:51:54 -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 02/10] powerpc/kernel/iommu: Align size for
 IOMMU_PAGE_SIZE on iommu_*_coherent()

On Sat, 2020-08-22 at 20:07 +1000, Alexey Kardashevskiy wrote:
> 
> On 18/08/2020 09:40, Leonardo Bras wrote:
> > Both iommu_alloc_coherent() and iommu_free_coherent() assume that once
> > size is aligned to PAGE_SIZE it will be aligned to IOMMU_PAGE_SIZE.
> 
> The only case when it is not aligned is when IOMMU_PAGE_SIZE > PAGE_SIZE
> which is unlikely but not impossible, we could configure the kernel for
> 4K system pages and 64K IOMMU pages I suppose. Do we really want to do
> this here, or simply put WARN_ON(tbl->it_page_shift > PAGE_SHIFT)?

I think it would be better to keep the code as much generic as possible
regarding page sizes. 

> Because if we want the former (==support), then we'll have to align the
> size up to the bigger page size when allocating/zeroing system pages,
> etc. 

This part I don't understand. Why do we need to align everything to the
bigger pagesize? 

I mean, is not that enough that the range [ret, ret + size[ is both
allocated by mm and mapped on a iommu range?

Suppose a iommu_alloc_coherent() of 16kB on PAGESIZE = 4k and
IOMMU_PAGE_SIZE() == 64k.
Why 4 * cpu_pages mapped by a 64k IOMMU page is not enough? 
All the space the user asked for is allocated and mapped for DMA.


> Bigger pages are not the case here as I understand it.

I did not get this part, what do you mean?

> > Update those functions to guarantee alignment with requested size
> > using IOMMU_PAGE_ALIGN() before doing iommu_alloc() / iommu_free().
> > 
> > Also, on iommu_range_alloc(), replace ALIGN(n, 1 << tbl->it_page_shift)
> > with IOMMU_PAGE_ALIGN(n, tbl), which seems easier to read.
> > 
> > Signed-off-by: Leonardo Bras <leobras.c@...il.com>
> > ---
> >  arch/powerpc/kernel/iommu.c | 17 +++++++++--------
> >  1 file changed, 9 insertions(+), 8 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
> > index 9704f3f76e63..d7086087830f 100644
> > --- a/arch/powerpc/kernel/iommu.c
> > +++ b/arch/powerpc/kernel/iommu.c
> > @@ -237,10 +237,9 @@ static unsigned long iommu_range_alloc(struct device *dev,
> >  	}
> >  
> >  	if (dev)
> > -		boundary_size = ALIGN(dma_get_seg_boundary(dev) + 1,
> > -				      1 << tbl->it_page_shift);
> > +		boundary_size = IOMMU_PAGE_ALIGN(dma_get_seg_boundary(dev) + 1, tbl);
> 
> Run checkpatch.pl, should complain about a long line.

It's 86 columns long, which is less than the new limit of 100 columns
Linus announced a few weeks ago. checkpatch.pl was updated too:
https://www.phoronix.com/scan.php?page=news_item&px=Linux-Kernel-Deprecates-80-Col


> 
> 
> >  	else
> > -		boundary_size = ALIGN(1UL << 32, 1 << tbl->it_page_shift);
> > +		boundary_size = IOMMU_PAGE_ALIGN(1UL << 32, tbl);
> >  	/* 4GB boundary for iseries_hv_alloc and iseries_hv_map */
> >  
> >  	n = iommu_area_alloc(tbl->it_map, limit, start, npages, tbl->it_offset,
> > @@ -858,6 +857,7 @@ void *iommu_alloc_coherent(struct device *dev, struct iommu_table *tbl,
> >  	unsigned int order;
> >  	unsigned int nio_pages, io_order;
> >  	struct page *page;
> > +	size_t size_io = size;
> >  
> >  	size = PAGE_ALIGN(size);
> >  	order = get_order(size);
> > @@ -884,8 +884,9 @@ void *iommu_alloc_coherent(struct device *dev, struct iommu_table *tbl,
> >  	memset(ret, 0, size);
> >  
> >  	/* Set up tces to cover the allocated range */
> > -	nio_pages = size >> tbl->it_page_shift;
> > -	io_order = get_iommu_order(size, tbl);
> > +	size_io = IOMMU_PAGE_ALIGN(size_io, tbl);
> > +	nio_pages = size_io >> tbl->it_page_shift;
> > +	io_order = get_iommu_order(size_io, tbl);
> >  	mapping = iommu_alloc(dev, tbl, ret, nio_pages, DMA_BIDIRECTIONAL,
> >  			      mask >> tbl->it_page_shift, io_order, 0);
> >  	if (mapping == DMA_MAPPING_ERROR) {
> > @@ -900,11 +901,11 @@ void iommu_free_coherent(struct iommu_table *tbl, size_t size,
> >  			 void *vaddr, dma_addr_t dma_handle)
> >  {
> >  	if (tbl) {
> > -		unsigned int nio_pages;
> > +		size_t size_io = IOMMU_PAGE_ALIGN(size, tbl);
> > +		unsigned int nio_pages = size_io >> tbl->it_page_shift;
> >  
> > -		size = PAGE_ALIGN(size);
> > -		nio_pages = size >> tbl->it_page_shift;
> >  		iommu_free(tbl, dma_handle, nio_pages);
> > +
> 
> Unrelated new line.

Will be removed. Thanks!

> 
> 
> >  		size = PAGE_ALIGN(size);
> >  		free_pages((unsigned long)vaddr, get_order(size));
> >  	}
> >