linux-kernel - Re: [RFC v2 28/32] x86/tdx: Make pages shared in ioremap()

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Date:   Wed, 26 May 2021 15:20:32 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Borislav Petkov <bp@...en8.de>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [RFC v2 28/32] x86/tdx: Make pages shared in ioremap()



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