hv_netvsc: Preserve contiguous PFN grouping in the page buffer array

Starting with commit dca5161f9bd0 ("hv_netvsc: Check status in
SEND_RNDIS_PKT completion message") in the 6.3 kernel, the Linux
driver for Hyper-V synthetic networking (netvsc) occasionally reports
"nvsp_rndis_pkt_complete error status: 2".[1] This error indicates
that Hyper-V has rejected a network packet transmit request from the
guest, and the outgoing network packet is dropped. Higher level
network protocols presumably recover and resend the packet so there is
no functional error, but performance is slightly impacted. Commit
dca5161f9bd0 is not the cause of the error -- it only added reporting
of an error that was already happening without any notice. The error
has presumably been present since the netvsc driver was originally
introduced into Linux.

The root cause of the problem is that the netvsc driver in Linux may
send an incorrectly formatted VMBus message to Hyper-V when
transmitting the network packet. The incorrect formatting occurs when
the rndis header of the VMBus message crosses a page boundary due to
how the Linux skb head memory is aligned. In such a case, two PFNs are
required to describe the location of the rndis header, even though
they are contiguous in guest physical address (GPA) space. Hyper-V
requires that two rndis header PFNs be in a single "GPA range" data
struture, but current netvsc code puts each PFN in its own GPA range,
which Hyper-V rejects as an error.

The incorrect formatting occurs only for larger packets that netvsc
must transmit via a VMBus "GPA Direct" message. There's no problem
when netvsc transmits a smaller packet by copying it into a pre-
allocated send buffer slot because the pre-allocated slots don't have
page crossing issues.

After commit 14ad6ed30a10 ("net: allow small head cache usage with
large MAX_SKB_FRAGS values") in the 6.14-rc4 kernel, the error occurs
much more frequently in VMs with 16 or more vCPUs. It may occur every
few seconds, or even more frequently, in an ssh session that outputs a
lot of text. Commit 14ad6ed30a10 subtly changes how skb head memory is
allocated, making it much more likely that the rndis header will cross
a page boundary when the vCPU count is 16 or more. The changes in
commit 14ad6ed30a10 are perfectly valid -- they just had the side
effect of making the netvsc bug more prominent.

Current code in init_page_array() creates a separate page buffer array
entry for each PFN required to identify the data to be transmitted.
Contiguous PFNs get separate entries in the page buffer array, and any
information about contiguity is lost.

Fix the core issue by having init_page_array() construct the page
buffer array to represent contiguous ranges rather than individual
pages. When these ranges are subsequently passed to
netvsc_build_mpb_array(), it can build GPA ranges that contain
multiple PFNs, as required to avoid the error "nvsp_rndis_pkt_complete
error status: 2". If instead the network packet is sent by copying
into a pre-allocated send buffer slot, the copy proceeds using the
contiguous ranges rather than individual pages, but the result of the
copying is the same. Also fix rndis_filter_send_request() to construct
a contiguous range, since it has its own page buffer array.

This change has a side benefit in CoCo VMs in that netvsc_dma_map()
calls dma_map_single() on each contiguous range instead of on each
page. This results in fewer calls to dma_map_single() but on larger
chunks of memory, which should reduce contention on the swiotlb.

Since the page buffer array now contains one entry for each contiguous
range instead of for each individual page, the number of entries in
the array can be reduced, saving 208 bytes of stack space in
netvsc_xmit() when MAX_SKG_FRAGS has the default value of 17.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=217503

Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217503
Cc: <stable@vger.kernel.org> # 6.1.x
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Link: https://patch.msgid.link/20250513000604.1396-4-mhklinux@outlook.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions