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

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Message-ID: <de96fb04-b8a4-02b4-be51-0e1c49976f22@amd.com>
Date:   Wed, 26 May 2021 17:14:05 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.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 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.

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;
>>
>>