[PATCH hyperv-next v2 1/4] Documentation: hyperv: Confidential VMBus

[ALOK TIWARI]

On 12-05-2025 04:37, Roman Kisel wrote:
> Define what the confidential VMBus is and describe what advantages
> it offers on the capable hardware.
> 
> Signed-off-by: Roman Kisel <romank at linux.microsoft.com>
> ---
>   Documentation/virt/hyperv/vmbus.rst | 41 +++++++++++++++++++++++++++++
>   1 file changed, 41 insertions(+)
> 
> diff --git a/Documentation/virt/hyperv/vmbus.rst b/Documentation/virt/hyperv/vmbus.rst
> index 1dcef6a7fda3..ca2b948e5070 100644
> --- a/Documentation/virt/hyperv/vmbus.rst
> +++ b/Documentation/virt/hyperv/vmbus.rst
> @@ -324,3 +324,44 @@ rescinded, neither Hyper-V nor Linux retains any state about
>   its previous existence. Such a device might be re-added later,
>   in which case it is treated as an entirely new device. See
>   vmbus_onoffer_rescind().
> +
> +Confidential VMBus
> +------------------
> +

The purpose and benefits of the Confidential VMBus are not clearly stated.
for example:
"Confidential VMBus provides a secure communication channel between 
guest and paravisor, ensuring that sensitive data is protected from 
hypervisor-level access through memory encryption and register state 
isolation."

> +The confidential VMBus provides the control and data planes where
> +the guest doesn't talk to either the hypervisor or the host. Instead,
> +it relies on the trusted paravisor. The hardware (SNP or TDX) encrypts
> +the guest memory and the register state also measuring the paravisor

s/alos/while and s/via using/using
"register state while measuring the paravisor image using the platform 
security"

> +image via using the platform security processor to ensure trusted and
> +confidential computing.
> +
> +To support confidential communication with the paravisor, a VMBus client
> +will first attempt to use regular, non-isolated mechanisms for communication.
> +To do this, it must:
> +
> +* Configure the paravisor SIMP with an encrypted page. The paravisor SIMP is
> +  configured by setting the relevant MSR directly, without using GHCB or tdcall.
> +
> +* Enable SINT 2 on both the paravisor and hypervisor, without setting the proxy
> +  flag on the paravisor SINT. Enable interrupts on the paravisor SynIC.
> +
> +* Configure both the paravisor and hypervisor event flags page.
> +  Both pages will need to be scanned when VMBus receives a channel interrupt.
> +
> +* Send messages to the paravisor by calling HvPostMessage directly, without using
> +  GHCB or tdcall.
> +
> +* Set the EOM MSR directly in the paravisor, without using GHCB or tdcall.
> +
> +If sending the InitiateContact message using non-isolated HvPostMessage fails,
> +the client must fall back to using the hypervisor synic, by using the GHCB/tdcall
> +as appropriate.
> +
> +To fall back, the client will have to reconfigure the following:
> +
> +* Configure the hypervisor SIMP with a host-visible page.
> +  Since the hypervisor SIMP is not used when in confidential mode,
> +  this can be done up front, or only when needed, whichever makes sense for
> +  the particular implementation.

"SIMP is not used in confidential mode,
this can be done either upfront or only when needed, depending on the 
specific implementation."

> +
> +* Set the proxy flag on SINT 2 for the paravisor.


Thanks,
Alok