linux-kernel - Re: [PATCHv6 27/30] x86/kvm: Make SWIOTLB buffer shared for TD guest

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Date:   Wed, 16 Mar 2022 15:24:30 -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: [PATCHv6 27/30] x86/kvm: Make SWIOTLB buffer shared for TD guest

On 3/15/22 19:08, 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.
> 
> Make SWIOTLB buffer shared by generalizing mem_encrypt_init() to cover
> TDX.

This could have saved me a trip to the console if it would have also said:

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

It's probably also worth noting:

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

The other adorable thing about this patch:

>  arch/x86/Kconfig                   | 2 +-
>  arch/x86/coco/core.c               | 1 +
>  arch/x86/include/asm/mem_encrypt.h | 6 +++---
>  arch/x86/mm/mem_encrypt.c          | 9 ++++++++-
>  4 files changed, 13 insertions(+), 5 deletions(-)

Is that it superficially has *zero* to do with SWIOTLB, also known as
the declared Subject.  It almost looks like this patch found a smaller
and weaker patch, beat it up, and stole its Subject.

I'd be fine with an "x86/mm" tag on this too.  I'm not sure what makes
it truly KVM-specific.

Can you send a revision with something a bit more descriptive, please?