linux-kernel - Re: [PATCH v9 5/6] x86/sev: Add SNP-specific unaccepted memory support

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Message-ID: <101c3910-a164-4b4c-9474-8743dc6d1199@cschramm.eu>
Date:   Wed, 6 Sep 2023 16:04:14 +0200
From:   Christopher Schramm <linux@...hramm.eu>
To:     Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v9 5/6] x86/sev: Add SNP-specific unaccepted memory
 support

> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 5c72067c06d4..b9c451f75d5e 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1543,11 +1543,13 @@ config X86_MEM_ENCRYPT
>   config AMD_MEM_ENCRYPT
>   	bool "AMD Secure Memory Encryption (SME) support"
>   	depends on X86_64 && CPU_SUP_AMD
> +	depends on EFI_STUB
>   	select DMA_COHERENT_POOL
>   	select ARCH_USE_MEMREMAP_PROT
>   	select INSTRUCTION_DECODER
>   	select ARCH_HAS_CC_PLATFORM
>   	select X86_MEM_ENCRYPT
> +	select UNACCEPTED_MEMORY
>   	help
>   	  Say yes to enable support for the encryption of system memory.
>   	  This requires an AMD processor that supports Secure Memory

Unfortunately this makes AMD_MEM_ENCRYPT depend on EFI just to 
unconditionally enable UNACCEPTED_MEMORY. It seems like an easy target 
to make that optional, e.g. with a separate configuration item:

---
config AMD_UNACCEPTED_MEMORY
        def_bool y
        depends on AMD_MEM_ENCRYPT && EFI_STUB
        select UNACCEPTED_MEMORY
---

Using that we can successfully build and run SNP VMs without UEFI/OVMF 
(which we already did with earlier Linux versions).

 From a quick look at

   [PATCHv14 9/9] x86/tdx: Add unaccepted memory support

it actually seems very similar for INTEL_TDX_GUEST.

Ideally UNACCEPTED_MEMORY would not assume EFI either, but the 
implementation actually clearly does.