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Date:   Thu, 19 Oct 2023 16:25:44 -0700
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References: <20231018173409.1871540-1-seanjc@google.com> <85e332bf-decf-427c-a2e5-95ab872d4ea6@arm.com>
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Subject: Re: [PATCH] swiotlb: Rewrite comment explaining why the source is
 preserved on DMA_FROM_DEVICE
From:   Sean Christopherson <seanjc@google.com>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        iommu@lists.linux.dev, linux-kernel@vger.kernel.org,
        Yan Zhao <yan.y.zhao@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On Wed, Oct 18, 2023, Robin Murphy wrote:
> On 2023-10-18 18:34, Sean Christopherson wrote:
> > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > index 01637677736f..e071415a75dc 100644
> > --- a/kernel/dma/swiotlb.c
> > +++ b/kernel/dma/swiotlb.c
> > @@ -1296,11 +1296,13 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
> >   		pool->slots[index + i].orig_addr = slot_addr(orig_addr, i);
> >   	tlb_addr = slot_addr(pool->start, index) + offset;
> >   	/*
> > -	 * When dir == DMA_FROM_DEVICE we could omit the copy from the orig
> > -	 * to the tlb buffer, if we knew for sure the device will
> > -	 * overwrite the entire current content. But we don't. Thus
> > -	 * unconditional bounce may prevent leaking swiotlb content (i.e.
> > -	 * kernel memory) to user-space.
> > +	 * When the device is writing memory, i.e. dir == DMA_FROM_DEVICE, copy
> > +	 * the original buffer to the TLB buffer before initiating DMA in order
> > +	 * to preserve the original's data if the device does a partial write,
> > +	 * i.e. if the device doesn't overwrite the entire buffer.  Preserving
> > +	 * the original data, even if it's garbage, is necessary to match
> > +	 * hardware behavior (use of swiotlb is supposed to be transparent) and
> 
> Super-nit: I think that last "and" is superfluous (i.e. unwritten memory not
> magically corrupting itself *is* the aforementioned hardware behaviour).

Ah yeah, agreed.  How about this?

	/*
	 * When the device is writing memory, i.e. dir == DMA_FROM_DEVICE, copy
	 * the original buffer to the TLB buffer before initiating DMA in order
	 * to preserve the original's data if the device does a partial write,
	 * i.e. if the device doesn't overwrite the entire buffer.  Preserving
	 * the original data, even if it's garbage, is necessary to match
	 * hardware behavior.  Use of swiotlb is supposed to be transparent,
	 * i.e. swiotlb must not corrupt memory by clobbering unwritten bytes.
	 */