Re: [PATCH net 2/5] hv_netvsc: Use vmbus_sendpacket_mpb_desc() to send VMBus messages

[Simon Horman <horms@kernel.org>]

On Mon, May 12, 2025 at 05:06:01PM -0700, mhkelley58@gmail.com wrote:
> From: Michael Kelley <mhklinux@outlook.com>
> 
> netvsc currently uses vmbus_sendpacket_pagebuffer() to send VMBus
> messages. This function creates a series of GPA ranges, each of which
> contains a single PFN. However, if the rndis header in the VMBus
> message crosses a page boundary, the netvsc protocol with the host
> requires that both PFNs for the rndis header must be in a single "GPA
> range" data structure, which isn't possible with
> vmbus_sendpacket_pagebuffer(). As the first step in fixing this, add a
> new function netvsc_build_mpb_array() to build a VMBus message with
> multiple GPA ranges, each of which may contain multiple PFNs. Use
> vmbus_sendpacket_mpb_desc() to send this VMBus message to the host.
> 
> There's no functional change since higher levels of netvsc don't
> maintain or propagate knowledge of contiguous PFNs. Based on its
> input, netvsc_build_mpb_array() still produces a separate GPA range
> for each PFN and the behavior is the same as with
> vmbus_sendpacket_pagebuffer(). But the groundwork is laid for a
> subsequent patch to provide the necessary grouping.
> 
> Cc: <stable@vger.kernel.org> # 6.1.x
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> ---
>  drivers/net/hyperv/netvsc.c | 50 +++++++++++++++++++++++++++++++++----
>  1 file changed, 45 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/hyperv/netvsc.c b/drivers/net/hyperv/netvsc.c
> index d6f5b9ea3109..6d1705f87682 100644
> --- a/drivers/net/hyperv/netvsc.c
> +++ b/drivers/net/hyperv/netvsc.c
> @@ -1055,6 +1055,42 @@ static int netvsc_dma_map(struct hv_device *hv_dev,
>  	return 0;
>  }
>  
> +/* Build an "array" of mpb entries describing the data to be transferred
> + * over VMBus. After the desc header fields, each "array" entry is variable
> + * size, and each entry starts after the end of the previous entry. The
> + * "offset" and "len" fields for each entry imply the size of the entry.
> + *
> + * The pfns are in HV_HYP_PAGE_SIZE, because all communication with Hyper-V
> + * uses that granularity, even if the system page size of the guest is larger.
> + * Each entry in the input "pb" array must describe a contiguous range of
> + * guest physical memory so that the pfns are sequential if the range crosses
> + * a page boundary. The offset field must be < HV_HYP_PAGE_SIZE.

Hi Michael,

Is there a guarantee that this constraint is met. And moreover, is there a
guarantee that all of the entries will fit in desc? I am slightly concerned
that there may be an overrun lurking here.

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