Re: [PATCH 1/1] OvmfPkg/AmdSev: introduce EMBED_GRUB=FALSE to skip including Grub image

["James Bottomley" <jejb@linux.ibm.com>]

On Wed, 2021-07-07 at 10:42 +0000, Dov Murik wrote:
> The AmdSevX64 target includes an embedded Grub image to support
> secure
> (measured) boot of confidential guests with encrypted root images.
> 
> However, it is sometimes convenient to build this target without an
> embedded Grub.  We introduce the EMBED_GRUB setting (defaults to
> TRUE),
> which conditions the generation (grub.sh) and inclusion of the Grub
> image.  Now building AmdSevX64 with -DEMBED_GRUB=FALSE allows it.
> 
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Ashish Kalra <ashish.kalra@amd.com>
> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: Erdem Aktas <erdemaktas@google.com>
> Cc: James Bottomley <jejb@linux.ibm.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Min Xu <min.m.xu@intel.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Tobin Feldman-Fitzthum <tobin@linux.ibm.com>
> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
> ---
>  OvmfPkg/AmdSev/AmdSevX64.dsc | 16 +++++++++++++++-
>  OvmfPkg/AmdSev/AmdSevX64.fdf |  2 ++
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/OvmfPkg/AmdSev/AmdSevX64.dsc
> b/OvmfPkg/AmdSev/AmdSevX64.dsc
> index 1d487befae08..ba7d6fe6b749 100644
> --- a/OvmfPkg/AmdSev/AmdSevX64.dsc
> +++ b/OvmfPkg/AmdSev/AmdSevX64.dsc
> @@ -25,7 +25,6 @@ [Defines]
>    BUILD_TARGETS                  = NOOPT|DEBUG|RELEASE
>    SKUID_IDENTIFIER               = DEFAULT
>    FLASH_DEFINITION               = OvmfPkg/AmdSev/AmdSevX64.fdf
> -  PREBUILD                       = sh OvmfPkg/AmdSev/Grub/grub.sh
>  
>    #
>    # Defines for default states.  These can be changed on the command
> line.
> @@ -40,6 +39,19 @@ [Defines]
>    #
>    DEFINE BUILD_SHELL             = FALSE
>  
> +  #
> +  # Embed Grub into the OVMF image so they are measured together
> when launching
> +  # confidential guest
> +  #
> +  DEFINE EMBED_GRUB              = TRUE
> +
> +!if $(EMBED_GRUB) == TRUE
> +  #
> +  # This step builds the grub.efi binary image if needed
> +  #
> +  PREBUILD                       = sh OvmfPkg/AmdSev/Grub/grub.sh
> +!endif
> +
>    #
>    # Device drivers
>    #
> @@ -784,7 +796,9 @@ [Components]
>    }
>  !endif
>    OvmfPkg/AmdSev/SecretDxe/SecretDxe.inf
> +!if $(EMBED_GRUB) == TRUE
>    OvmfPkg/AmdSev/Grub/Grub.inf
> +!endif
>  !if $(BUILD_SHELL) == TRUE
>    ShellPkg/Application/Shell/Shell.inf {
>      <LibraryClasses>
> diff --git a/OvmfPkg/AmdSev/AmdSevX64.fdf
> b/OvmfPkg/AmdSev/AmdSevX64.fdf
> index 9977b0f00a18..ee3d96bb813f 100644
> --- a/OvmfPkg/AmdSev/AmdSevX64.fdf
> +++ b/OvmfPkg/AmdSev/AmdSevX64.fdf
> @@ -270,7 +270,9 @@ [FV.DXEFV]
>  INF  OvmfPkg/LinuxInitrdDynamicShellCommand/LinuxInitrdDynamicShellC
> ommand.inf
>  !endif
>  INF OvmfPkg/AmdSev/SecretDxe/SecretDxe.inf
> +!if $(EMBED_GRUB) == TRUE
>  INF  OvmfPkg/AmdSev/Grub/Grub.inf
> +!endif
>  !if $(BUILD_SHELL) == TRUE
>  INF  ShellPkg/Application/Shell/Shell.inf
>  !endif

This likely isn't enough: the boot pathway of the AmdSev package is
stripped down and designed to fail if grub won't boot, so if you set
EMBED_GRUB = false, you'll likely build a system that won't boot.  This
would still work for the Kata use case, if the kernel and initrd are
plumbed back in, but it won't work for the generic use case.  I think
the change log needs to describe the use cases so we don't end up
getting a load of annoyed people building systems that won't work for
them.

There's also the broader question of whether this should all be
integrated back into OvmfX64Pkg with more determination done at
runtime, so we can build fewer separate binaries?

James