On 5/26/21 3:14 PM, Tom Lendacky wrote:
> On 5/26/21 5:02 PM, Tom Lendacky wrote:
>> On 5/26/21 4:37 PM, Kuppuswamy, Sathyanarayanan wrote:
>>>
>>>
>>> On 5/21/21 9:19 AM, Tom Lendacky wrote:
>>>> In arch/x86/mm/mem_encrypt.c, sme_early_init() (should have renamed that
>>>> when SEV support was added), we do:
>>>>      if (sev_active())
>>>>          swiotlb_force = SWIOTLB_FORCE;
>>>>
>>>> TDX should be able to do a similar thing without having to touch
>>>> arch/x86/kernel/pci-swiotlb.c.
>>>>
>>>> That would remove any confusion over SME being part of a
>>>> protected_guest_has() call.
>>>
>>> You mean sme_active() check in arch/x86/kernel/pci-swiotlb.c is redundant?
>>
>> No, the sme_active() check is required to make sure that SWIOTLB is
>> available under SME. Encrypted DMA is supported under SME if the device
>> supports 64-bit DMA. But if the device doesn't support 64-bit DMA and the
>> IOMMU is not active, then DMA will be bounced through SWIOTLB.
>>
>> As compared to SEV, where all DMA has to be bounced through SWIOTLB or
>> unencrypted memory. For that, swiotlb_force is used.
> 
> I should probably add that SME is memory encryption support for
> host/hypervisor/bare-metal, while SEV is memory encryption support for
> virtualization.

Got it. Thanks for clarification.

> 
> Thanks,
> Tom
> 
>>
>> Thanks,
>> Tom
>>
>>>
>>>   41 int __init pci_swiotlb_detect_4gb(void)
>>>   42 {
>>>   43         /* don't initialize swiotlb if iommu=off (no_iommu=1) */
>>>   44         if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN)
>>>   45                 swiotlb = 1;
>>>   46
>>>   47         /*
>>>   48          * If SME is active then swiotlb will be set to 1 so that bounce
>>>   49          * buffers are allocated and used for devices that do not support
>>>   50          * the addressing range required for the encryption mask.
>>>   51          */
>>>   52         if (sme_active() || is_tdx_guest())
>>>   53                 swiotlb = 1;
>>>
>>>

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer