Re: [PATCH v3] dmadev: fix structure alignment

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Thu, Mar 21, 2024 at 11:06:34AM +0100, Thomas Monjalon wrote:
> 21/03/2024 10:18, Ma, WenwuX:
> > From: Thomas Monjalon <thomas@monjalon.net>
> > > 21/03/2024 02:25, Ma, WenwuX:
> > > > From: Thomas Monjalon <thomas@monjalon.net>
> > > > > 20/03/2024 08:23, Wenwu Ma:
> > > > > > The structure rte_dma_dev needs to be aligned to the cache line,
> > > > > > but the return value of malloc may not be aligned to the cache
> > > > > > line. When we use memset to clear the rte_dma_dev object, it may
> > > > > > cause a segmentation fault in clang-x86-platform.
> > > > > >
> > > > > > This is because clang uses the "vmovaps" assembly instruction for
> > > > > > memset, which requires that the operands (rte_dma_dev objects)
> > > > > > must aligned on a 16-byte boundary or a general-protection
> > > > > > exception (#GP) is generated.
> > > > > >
> > > > > > Therefore, either additional memory is applied for re-alignment,
> > > > > > or the rte_dma_dev object does not require cache line alignment.
> > > > > > The patch chooses the former option to fix the issue.
> > > > > >
> > > > > > Fixes: b36970f2e13e ("dmadev: introduce DMA device library")
> > > > > > Cc: stable@dpdk.org
> > > > > >
> > > > > > Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
> > > > > [..]
> > > > > > -	size = dma_devices_max * sizeof(struct rte_dma_dev);
> > > > > > -	rte_dma_devices = malloc(size);
> > > > > > -	if (rte_dma_devices == NULL)
> > > > > > +	/* The dma device object is expected to align cacheline, but
> > > > > > +	 * the return value of malloc may not be aligned to the cache line.
> > > > > > +	 * Therefore, extra memory is applied for realignment.
> > > > > > +	 * note: We do not call posix_memalign/aligned_alloc because it is
> > > > > > +	 * version dependent on libc.
> > > > > > +	 */
> > > > > > +	size = dma_devices_max * sizeof(struct rte_dma_dev) +
> > > > > > +		RTE_CACHE_LINE_SIZE;
> > > > > > +	ptr = malloc(size);
> > > > > > +	if (ptr == NULL)
> > > > > >  		return -ENOMEM;
> > > > > > -	memset(rte_dma_devices, 0, size);
> > > > > > +	memset(ptr, 0, size);
> > > > > > +
> > > > > > +	rte_dma_devices = RTE_PTR_ALIGN(ptr, RTE_CACHE_LINE_SIZE);
> > > > >
> > > > > Why not using aligned_alloc()?
> > > > > https://en.cppreference.com/w/c/memory/aligned_alloc
> > > > >
> > > > >
> > > > because it is version dependent on libc.
> > > 
> > > Which libc is required?
> > > 
> > 
> > using the 'man aligned_alloc' command, we has the following description:
> > 
> > VERSIONS
> >        The functions memalign(), valloc(), and pvalloc() have been available in all Linux libc libraries.
> > 
> >        The function aligned_alloc() was added to glibc in version 2.16.
> 
> released in 2012-06-30

If we are using C11 we probably already implicitly depend on the glibc
that supports aligned_alloc (introduced in C11).

> 
> >        The function posix_memalign() is available since glibc 2.1.91.
> 
> I think we could bump our libc requirements for these functions.
> 
> I understand there is also a concern on Windows,
> but an alternative exists there.
> We may need a wrapper like "rte_alloc_align".

Yes, I'm afraid we would probably have to introduce
rte_aligned_alloc/rte_aligned_free. On Windows this would simply
forward to _aligned_alloc() and _aligned_free() respectively.

ty