linux-kernel - RE: [PATCH hyperv-next v3 07/15] Drivers: hv: Post messages via the confidential VMBus if available

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Message-ID:
 <SN6PR02MB4157E23F495276EA814C775BD472A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Wed, 18 Jun 2025 16:18:40 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Roman Kisel <romank@...ux.microsoft.com>, "alok.a.tiwari@...cle.com"
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Subject: RE: [PATCH hyperv-next v3 07/15] Drivers: hv: Post messages via the
 confidential VMBus if available

From: Roman Kisel <romank@...ux.microsoft.com> Sent: Tuesday, June 3, 2025 5:44 PM
> 
> When the confidential VMBus is available, the guest should post
> messages via the paravisor.
> 
> Update hv_post_message() to request posting messages from the paravisor

"via the paravisor"?  I'm not sure what "from the paravisor" means. And
you used "via" in the previous sentence and patch Subject.

> rather than through GHCB or TD calls.
> 
> Signed-off-by: Roman Kisel <romank@...ux.microsoft.com>
> ---
>  drivers/hv/hv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index e25c91eb6af5..1f7cf1244509 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -74,7 +74,7 @@ int hv_post_message(union hv_connection_id connection_id,
>  	aligned_msg->payload_size = payload_size;
>  	memcpy((void *)aligned_msg->payload, payload, payload_size);
> 
> -	if (ms_hyperv.paravisor_present) {
> +	if (ms_hyperv.paravisor_present && !vmbus_is_confidential()) {

Does this change make post_msg_page unnecessary when Confidential
VMBus is present? When using Confidential VMBus, the code path will be
to use a normal hypercall, which will go to the paravisor, and hence
doesn't need decrypted memory.

If my thinking is correct, the code in hv_synic_alloc() could be updated to
not allocate post_msg_page when vmbus_is_confidential().

>  		if (hv_isolation_type_tdx())
>  			status = hv_tdx_hypercall(HVCALL_POST_MESSAGE,
>  						  virt_to_phys(aligned_msg), 0);
> --
> 2.43.0