linux-kernel - Re: [PATCH 1/2] xen: swiotlb: Use swiotlb bouncing if kmalloc allocation demands it

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Message-ID: <alpine.DEB.2.22.394.2505020955140.3879245@ubuntu-linux-20-04-desktop>
Date: Fri, 2 May 2025 10:07:36 -0700 (PDT)
From: Stefano Stabellini <sstabellini@...nel.org>
To: John Ernberg <john.ernberg@...ia.se>
cc: Juergen Gross <jgross@...e.com>, 
    Stefano Stabellini <sstabellini@...nel.org>, 
    Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>, 
    Catalin Marinas <catalin.marinas@....com>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>, 
    "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, 
    "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
    "imx@...ts.linux.dev" <imx@...ts.linux.dev>, 
    "stable@...nel.org" <stable@...nel.org>
Subject: Re: [PATCH 1/2] xen: swiotlb: Use swiotlb bouncing if kmalloc
 allocation demands it

On Fri, 2 May 2025, John Ernberg wrote:
> Xen swiotlb support was missed when the patch set starting with
> 4ab5f8ec7d71 ("mm/slab: decouple ARCH_KMALLOC_MINALIGN from
> ARCH_DMA_MINALIGN") was merged.
> 
> When running Xen on iMX8QXP, a SoC without IOMMU, the effect was that USB
> transfers ended up corrupted when there was more than one URB inflight at
> the same time.
> 
> Add a call to dma_kmalloc_needs_bounce() to make sure that allocations too
> small for DMA get bounced via swiotlb.
> 
> Closes: https://lore.kernel.org/linux-usb/ab2776f0-b838-4cf6-a12a-c208eb6aad59@actia.se/
> Fixes: 4ab5f8ec7d71 ("mm/slab: decouple ARCH_KMALLOC_MINALIGN from ARCH_DMA_MINALIGN")
> Cc: stable@...nel.org # v6.5+
> Signed-off-by: John Ernberg <john.ernberg@...ia.se>

Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>


> ---
> 
> It's impossible to pick an exact fixes tag since this driver was missed
> in the flagged patch set. I picked one I felt gave a decent enough picture
> for someone coming across this later.
> ---
>  drivers/xen/swiotlb-xen.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index 1f65795cf5d7..ef56a2500ed6 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -217,6 +217,7 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
>  	 * buffering it.
>  	 */
>  	if (dma_capable(dev, dev_addr, size, true) &&
> +	    !dma_kmalloc_needs_bounce(dev, size, dir) &&
>  	    !range_straddles_page_boundary(phys, size) &&
>  		!xen_arch_need_swiotlb(dev, phys, dev_addr) &&
>  		!is_swiotlb_force_bounce(dev))
> -- 
> 2.49.0
>