Re: [PATCH] swiotlb: Rewrite comment explaining why the source is preserved on DMA_FROM_DEVICE

[Sean Christopherson <seanjc@google.com>]

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.
	 */