Re: [PATCH v2 2/3] docs: Add SEV-ES documentation to amd-memory-encryption.txt

[Laszlo Ersek <lersek@redhat.com>]

On 04/23/21 22:08, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> Update the amd-memory-encryption.txt file with information about SEV-ES,
> including how to launch an SEV-ES guest and some of the differences
> between SEV and SEV-ES guests in regards to launching and measuring the
> guest.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  docs/amd-memory-encryption.txt | 54 +++++++++++++++++++++++++++++-----
>  1 file changed, 47 insertions(+), 7 deletions(-)
> 
> diff --git a/docs/amd-memory-encryption.txt b/docs/amd-memory-encryption.txt
> index ed85159ea7..ffca382b5f 100644
> --- a/docs/amd-memory-encryption.txt
> +++ b/docs/amd-memory-encryption.txt
> @@ -15,6 +15,13 @@ includes commands for launching, snapshotting, migrating and debugging the
>  encrypted guest. These SEV commands can be issued via KVM_MEMORY_ENCRYPT_OP
>  ioctls.
>  
> +Secure Encrypted Virtualization - Encrypted State (SEV-ES) builds on the SEV
> +support to additionally protect the guest register state. In order to allow a
> +hypervisor to perform functions on behalf of a guest, there is architectural
> +support for notifying a guest's operating system when certain types of VMEXITs
> +are about to occur. This allows the guest to selectively share information with
> +the hypervisor to satisfy the requested function.
> +
>  Launching
>  ---------
>  Boot images (such as bios) must be encrypted before a guest can be booted. The
> @@ -24,6 +31,9 @@ together generate a fresh memory encryption key for the VM, encrypt the boot
>  images and provide a measurement than can be used as an attestation of a
>  successful launch.
>  
> +For a SEV-ES guest, the LAUNCH_UPDATE_VMSA command is also used to encrypt the
> +guest register state, or VM save area (VMSA), for all of the guest vCPUs.
> +
>  LAUNCH_START is called first to create a cryptographic launch context within
>  the firmware. To create this context, guest owner must provide a guest policy,
>  its public Diffie-Hellman key (PDH) and session parameters. These inputs
> @@ -40,6 +50,12 @@ The guest policy can be provided via the 'policy' property (see below)
>  # ${QEMU} \
>     sev-guest,id=sev0,policy=0x1...\
>  
> +Setting the "SEV-ES required" policy bit (bit 2) will launch the guest as a
> +SEV-ES guest (see below)
> +
> +# ${QEMU} \
> +   sev-guest,id=sev0,policy=0x5...\
> +
>  The guest owner provided DH certificate and session parameters will be used to
>  establish a cryptographic session with the guest owner to negotiate keys used
>  for the attestation.
> @@ -55,13 +71,19 @@ created via the LAUNCH_START command. If required, this command can be called
>  multiple times to encrypt different memory regions. The command also calculates
>  the measurement of the memory contents as it encrypts.
>  
> -LAUNCH_MEASURE can be used to retrieve the measurement of encrypted memory.
> -This measurement is a signature of the memory contents that can be sent to the
> -guest owner as an attestation that the memory was encrypted correctly by the
> -firmware. The guest owner may wait to provide the guest confidential information
> -until it can verify the attestation measurement. Since the guest owner knows the
> -initial contents of the guest at boot, the attestation measurement can be
> -verified by comparing it to what the guest owner expects.
> +LAUNCH_UPDATE_VMSA encrypts all the vCPU VMSAs for a SEV-ES guest using the
> +cryptographic context created via the LAUNCH_START command. The command also
> +calculates the measurement of the VMSAs as it encrypts them.
> +
> +LAUNCH_MEASURE can be used to retrieve the measurement of encrypted memory and,
> +for a SEV-ES guest, encrypted VMSAs. This measurement is a signature of the
> +memory contents and, for a SEV-ES guest, the VMSA contents, that can be sent
> +to the guest owner as an attestation that the memory and VMSAs were encrypted
> +correctly by the firmware. The guest owner may wait to provide the guest
> +confidential information until it can verify the attestation measurement.
> +Since the guest owner knows the initial contents of the guest at boot, the
> +attestation measurement can be verified by comparing it to what the guest owner
> +expects.
>  
>  LAUNCH_FINISH finalizes the guest launch and destroys the cryptographic
>  context.
> @@ -75,6 +97,22 @@ To launch a SEV guest
>      -machine ...,confidential-guest-support=sev0 \
>      -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1
>  
> +To launch a SEV-ES guest
> +
> +# ${QEMU} \
> +    -machine ...,confidential-guest-support=sev0 \
> +    -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1,policy=0x5
> +
> +An SEV-ES guest has some restrictions as compared to a SEV guest. Because the
> +guest register state is encrypted and cannot be updated by the VMM/hypervisor,
> +a SEV-ES guest:
> + - Does not support SMM - SMM support requires updating the guest register
> +   state.
> + - Does not support reboot - a system reset requires updating the guest register
> +   state.
> + - Requires in-kernel irqchip - the burden is placed on the hypervisor to
> +   manage booting APs.
> +
>  Debugging
>  -----------
>  Since the memory contents of a SEV guest are encrypted, hypervisor access to
> @@ -101,8 +139,10 @@ Secure Encrypted Virtualization Key Management:
>  
>  KVM Forum slides:
>  http://www.linux-kvm.org/images/7/74/02x08A-Thomas_Lendacky-AMDs_Virtualizatoin_Memory_Encryption_Technology.pdf
> +https://www.linux-kvm.org/images/9/94/Extending-Secure-Encrypted-Virtualization-with-SEV-ES-Thomas-Lendacky-AMD.pdf
>  
>  AMD64 Architecture Programmer's Manual:
>     http://support.amd.com/TechDocs/24593.pdf
>     SME is section 7.10
>     SEV is section 15.34
> +   SEV-ES is section 15.35
> 

Looks good to me.

Acked-by: Laszlo Ersek <lersek@redhat.com>