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From: Thomas Monjalon <thomas@monjalon.net>
To: "fengchengwen@huawei.com" <fengchengwen@huawei.com>,
"Ma, WenwuX" <wenwux.ma@intel.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>, "Jiale, SongX" <songx.jiale@intel.com>,
"stable@dpdk.org" <stable@dpdk.org>, Tyler Retzlaff <roretzla@microsoft.com>,
david.marchand@redhat.com, bruce.richardson@intel.com
Subject: Re: [PATCH v3] dmadev: fix structure alignment
Date: Thu, 21 Mar 2024 11:06:34 +0100
Message-ID: <7851657.gsGJI6kyIV@thomas>
In-Reply-To: <MN0PR11MB61108C052D9508FCD5092EA2EB322@MN0PR11MB6110.namprd11.prod.outlook.com>
References: <20240308053711.1260154-1-wenwux.ma@intel.com>
<2631241.9Mp67QZiUf@thomas>
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Errors-To: dev-bounces@dpdk.org
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
> 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".