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

[Lendacky, Thomas]

On 7/24/19 12:06 PM, Robin Murphy wrote:
> 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)
>>>> Â }
>>>> Â EXPORT_SYMBOL(sev_active);
>>>> Â +/* Override for DMA direct allocation check -
>>>> ARCH_HAS_FORCE_DMA_UNENCRYPTED */
>>>> +bool force_dma_unencrypted(struct device *dev)
>>>> +{
>>>> +ÂÂÂ /*
>>>> +ÂÂÂÂ * For SEV, all DMA must be to unencrypted addresses.
>>>> +ÂÂÂÂ */
>>>> +ÂÂÂ if (sev_active())
>>>> +ÂÂÂÂÂÂÂ return true;
>>>> +
>>>> +ÂÂÂ /*
>>>> +ÂÂÂÂ * For SME, all DMA must be to unencrypted addresses if the
>>>> +ÂÂÂÂ * device does not support DMA to addresses that include the
>>>> +ÂÂÂÂ * encryption mask.
>>>> +ÂÂÂÂ */
>>>> +ÂÂÂ if (sme_active()) {
>>>> +ÂÂÂÂÂÂÂ u64 dma_enc_mask = DMA_BIT_MASK(__ffs64(sme_me_mask));
>>>> +ÂÂÂÂÂÂÂ u64 dma_dev_mask = min_not_zero(dev->coherent_dma_mask,
>>>> +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ dev->bus_dma_mask);
>>>> +
>>>> +ÂÂÂÂÂÂÂ if (dma_dev_mask <= dma_enc_mask)
>>>> +ÂÂÂÂÂÂÂÂÂÂÂ 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:
> 
> ÂÂÂÂif (sme_me_mask & dma_dev_mask == sme_me_mask)

Actually !=, but yes, it could have been done like that, I just didn't
think of it.

Thanks,
Tom

> 
> ?
> 
> Robin.