linux-kernel - Re: [PATCH v2 4/8] dma-mapping: Separate DMA sync issuing and completion waiting

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251227200706.GN11869@unreal>
Date: Sat, 27 Dec 2025 22:07:06 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Barry Song <21cnbao@...il.com>
Cc: catalin.marinas@....com, m.szyprowski@...sung.com, robin.murphy@....com,
	will@...nel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xenproject.org, Barry Song <baohua@...nel.org>,
	Ada Couprie Diaz <ada.coupriediaz@....com>,
	Ard Biesheuvel <ardb@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Ryan Roberts <ryan.roberts@....com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Joerg Roedel <joro@...tes.org>, Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
	Tangquan Zheng <zhengtangquan@...o.com>
Subject: Re: [PATCH v2 4/8] dma-mapping: Separate DMA sync issuing and
 completion waiting

On Sat, Dec 27, 2025 at 11:52:44AM +1300, Barry Song wrote:
> From: Barry Song <baohua@...nel.org>
> 
> Currently, arch_sync_dma_for_cpu and arch_sync_dma_for_device
> always wait for the completion of each DMA buffer. That is,
> issuing the DMA sync and waiting for completion is done in a
> single API call.
> 
> For scatter-gather lists with multiple entries, this means
> issuing and waiting is repeated for each entry, which can hurt
> performance. Architectures like ARM64 may be able to issue all
> DMA sync operations for all entries first and then wait for
> completion together.
> 
> To address this, arch_sync_dma_for_* now issues DMA operations in
> batch, followed by a flush. On ARM64, the flush is implemented
> using a dsb instruction within arch_sync_dma_flush().
> 
> For now, add arch_sync_dma_flush() after each
> arch_sync_dma_for_*() call. arch_sync_dma_flush() is defined as a
> no-op on all architectures except arm64, so this patch does not
> change existing behavior. Subsequent patches will introduce true
> batching for SG DMA buffers.
> 
> Cc: Leon Romanovsky <leon@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Ada Couprie Diaz <ada.coupriediaz@....com>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: Marc Zyngier <maz@...nel.org>
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Stefano Stabellini <sstabellini@...nel.org>
> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
> Cc: Tangquan Zheng <zhengtangquan@...o.com>
> Signed-off-by: Barry Song <baohua@...nel.org>
> ---
>  arch/arm64/include/asm/cache.h |  6 ++++++
>  arch/arm64/mm/dma-mapping.c    |  4 ++--
>  drivers/iommu/dma-iommu.c      | 37 +++++++++++++++++++++++++---------
>  drivers/xen/swiotlb-xen.c      | 24 ++++++++++++++--------
>  include/linux/dma-map-ops.h    |  6 ++++++
>  kernel/dma/direct.c            |  8 ++++++--
>  kernel/dma/direct.h            |  9 +++++++--
>  kernel/dma/swiotlb.c           |  4 +++-
>  8 files changed, 73 insertions(+), 25 deletions(-)

<...>

> +#ifndef arch_sync_dma_flush
> +static inline void arch_sync_dma_flush(void)
> +{
> +}
> +#endif

Over the weekend I realized a useful advantage of the ARCH_HAVE_* config
options: they make it straightforward to inspect the entire DMA path simply
by looking at the .config.

Thanks,
Reviewed-by: Leon Romanovsky <leonro@...dia.com>