Re: [PATCH] dma-direct: Force unencrypted DMA under SME for certain DMA masks

[Robin Murphy <robin.murphy@arm.com>]

On 24/07/2019 17:42, Lendacky, Thomas wrote:
> On 7/24/19 10:55 AM, Kirill A. Shutemov wrote:
>> On Wed, Jul 10, 2019 at 07:01:19PM +0000, Lendacky, Thomas wrote:
>>> @@ -351,6 +355,32 @@ bool sev_active(void)
>>>   }
>>>   EXPORT_SYMBOL(sev_active);
>>>   
>>> +/* Override for DMA direct allocation check - ARCH_HAS_FORCE_DMA_UNENCRYPTED */
>>> +bool force_dma_unencrypted(struct device *dev)
>>> +{
>>> +	/*
>>> +	 * For SEV, all DMA must be to unencrypted addresses.
>>> +	 */
>>> +	if (sev_active())
>>> +		return true;
>>> +
>>> +	/*
>>> +	 * For SME, all DMA must be to unencrypted addresses if the
>>> +	 * device does not support DMA to addresses that include the
>>> +	 * encryption mask.
>>> +	 */
>>> +	if (sme_active()) {
>>> +		u64 dma_enc_mask = DMA_BIT_MASK(__ffs64(sme_me_mask));
>>> +		u64 dma_dev_mask = min_not_zero(dev->coherent_dma_mask,
>>> +						dev->bus_dma_mask);
>>> +
>>> +		if (dma_dev_mask <= dma_enc_mask)
>>> +			return true;
>>
>> Hm. What is wrong with the dev mask being equal to enc mask? IIUC, it
>> means that device mask is wide enough to cover encryption bit, doesn't it?
> 
> Not really...  it's the way DMA_BIT_MASK works vs bit numbering. Let's say
> that sme_me_mask has bit 47 set. __ffs64 returns 47 and DMA_BIT_MASK(47)
> will generate a mask without bit 47 set (0x7fffffffffff). So the check
> will catch anything that does not support at least 48-bit DMA.

Couldn't that be expressed as just:

	if (sme_me_mask & dma_dev_mask == sme_me_mask)

?

Robin.
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