linux-kernel - Re: [PATCH] swiotlb: Rewrite comment explaining why the source is preserved on DMA_FROM

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Message-ID: <efab630e-e8b4-4011-b3ce-0dfcab984bcd@arm.com>
Date:   Fri, 20 Oct 2023 10:51:12 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Yan Zhao <yan.y.zhao@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] swiotlb: Rewrite comment explaining why the source is
 preserved on DMA_FROM_DEVICE

On 2023-10-20 00:25, Sean Christopherson wrote:
> 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.
> 	 */

Nice, that reads even more clearly IMO.

Cheers,
Robin.