linux-kernel - Re: [PATCH v5 01/16] x86/mm: Move force_dma

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Date:   Wed, 20 Oct 2021 09:43:43 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Richard Henderson <rth@...ddle.net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        James E J Bottomley <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        "David S . Miller" <davem@...emloft.net>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter H Anvin <hpa@...or.com>, Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v5 01/16] x86/mm: Move force_dma_unencrypted() to common
 code



On 10/20/21 9:11 AM, Tom Lendacky wrote:
>> Intel TDX doesn't allow VMM to access guest private memory. Any memory
>> that is required for communication with VMM must be shared explicitly
>> by setting the bit in page table entry. After setting the shared bit,
>> the conversion must be completed with MapGPA hypercall. Details about
>> MapGPA hypercall can be found in [1], sec 3.2.
>>
>> The call informs VMM about the conversion between private/shared
>> mappings. The shared memory is similar to unencrypted memory in AMD
>> SME/SEV terminology but the underlying process of sharing/un-sharing
>> the memory is different for Intel TDX guest platform.
>>
>> SEV assumes that I/O devices can only do DMA to "decrypted" physical
>> addresses without the C-bit set. In order for the CPU to interact with
>> this memory, the CPU needs a decrypted mapping. To add this support,
>> AMD SME code forces force_dma_unencrypted() to return true for
>> platforms that support AMD SEV feature. It will be used for DMA memory
>> allocation API to trigger set_memory_decrypted() for platforms that
>> support AMD SEV feature.
>>
>> TDX is similar. So, to communicate with I/O devices, related pages need
>> to be marked as shared. As mentioned above, shared memory in TDX
>> architecture is similar to decrypted memory in AMD SME/SEV. So similar
>> to AMD SEV, force_dma_unencrypted() has to forced to return true. This
>> support is added in other patches in this series.
>>
>> So move force_dma_unencrypted() out of AMD specific code and call AMD
>> specific (amd_force_dma_unencrypted()) initialization function from it.
>> force_dma_unencrypted() will be modified by later patches to include
>> Intel TDX guest platform specific initialization.
>>
>> Also, introduce new config option X86_MEM_ENCRYPT_COMMON that has to be
>> selected by all x86 memory encryption features. This will be selected
>> by both AMD SEV and Intel TDX guest config options.
>>
>> This is preparation for TDX changes in DMA code and it has no
>> functional change.
> 
> Can force_dma_unencrypted() be moved to arch/x86/kernel/cc_platform.c, 
> instead of creating a new file? It might fit better with patch #6.

Please check the final version of mem_encrypt_common.c

https://github.com/intel/tdx/blob/guest/arch/x86/mm/mem_encrypt_common.c

I am not sure whether it is alright to move mem_encrypt_init() and
arch_has_restricted_virtio_memory_access() to cc_platform.c

If this is fine, I can get rid of mem_encrypt_common.c

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer