linux-kernel - Re: [PATCHv7 27/30] x86/mm: Make DMA memory shared for TD guest

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Date:   Fri, 18 Mar 2022 08:53:27 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 27/30] x86/mm: Make DMA memory shared for TD guest

On 3/18/22 08:30, Kirill A. Shutemov wrote:
> Intel TDX doesn't allow VMM to directly access guest private memory.
> Any memory that is required for communication with the VMM must be
> shared explicitly. The same rule applies for any DMA to and from the
> TDX guest. All DMA pages have to be marked as shared pages. A generic way
> to achieve this without any changes to device drivers is to use the
> SWIOTLB framework.
> 
> In TDX guest, CC_ATTR_GUEST_MEM_ENCRYPT is set. It makes all DMA to be
> rerouted via SWIOTLB (see pci_swiotlb_detect()). mem_encrypt_init()
> generalized to cover TDX. It makes SWIOTLB buffer shared.

It would be nice to have one transition paragraph to link the last one
to this:

The previous patch ("Add support for TDX shared memory") gave TDX guests
the _ability_ to make some pages shared, but did not actually make any
pages shared.  This actually marks SWIOTLB buffers *as* shared.

Start returning true for cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) in
TDX guests.  This has several implications:

 * Allows the existing mem_encrypt_init() to be used for TDX which
   sets SWIOTLB buffers shared (aka. "decrypted").
 * Ensures that all DMA is routed via the SWIOTLB mechanism (see
   pci_swiotlb_detect())


> Stop selecting DYNAMIC_PHYSICAL_MASK directly. It will get set
> indirectly by selcting X86_MEM_ENCRYPT.

		 ^ selecting

> mem_encrypt_init() is currently under an AMD-specific #ifdef. Move it to
> a generic area of the header.

That new paragraph was kinda funky.  With the changelog improvements above:

Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>