Define what the confidential VMBus is and describe what advantages
it offers on the capable hardware.

Signed-off-by: Roman Kisel <romank@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..f600e3d09800 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@@ -324,3 +324,44 @@ rescinded, neither Hyper-V nor Linux retains any state about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 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
+image via using the platform security processor to ensure trsuted 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.
+
+* Set the proxy flag on SINT 2 for the paravisor.
-- 
2.43.0

On Tue, Apr 08, 2025 at 05:08:30PM -0700, 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@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..f600e3d09800 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 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
> +image via using the platform security processor to ensure trsuted and
> +confidential computing.
> +
> +To support confidential communication with the paravisor, a VmBus client

Please be consistent. In this document I see VMBus and VmBus. We should
stick with only one form.

Wei.

On 09-04-2025 05:38, Roman Kisel wrote:
>   in which case it is treated as an entirely new device. See
>   vmbus_onoffer_rescind().
> +
> +Confidential VMBus
> +------------------
> +
> +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
> +image via using the platform security processor to ensure trsuted and
> +confidential computing.
> +

typo trsuted  -> trusted

> +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:


Thanks,
Alok

On 4/10/2025 9:54 AM, ALOK TIWARI wrote:
> 
> 

[...]

> 
> typo trsuted  -> trusted
> 
>> +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:
> 

Thanks for your help with this patch, and the other ones!! I'll make
sure to use the spellchecker on the patches before sending another
version out.

> 
> Thanks,
> Alok

-- 
Thank you,
Roman