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Subject: Re: [virtio-comment] [PATCH v2] virtio-vsock: add max payload size config field

On 4/15/22 16:06, Stefano Garzarella wrote:
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On Wed, Apr 13, 2022 at 01:01:40PM +0300, Laura Loghin wrote:
Added a new field to the vsock device config space that
is limiting the size of the packet payload. This way
the driver is not allowed to allocate huge buffers, and
potentially fill up the entire memory.
Also defined a new feature bit for this, VIRTIO_VSOCK_F_SIZE_MAX,
in order to keep backwards compatibility.

Signed-off-by: Laura Loghin <lauralg@amazon.com>
virtio-vsock.tex | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)

diff --git a/virtio-vsock.tex b/virtio-vsock.tex
index d79984d..e6903bd 100644
--- a/virtio-vsock.tex
+++ b/virtio-vsock.tex
@@ -23,6 +23,10 @@ \subsection{Feature bits}\label{sec:Device Types / Socket Device / Feature bits}
\item[VIRTIO_VSOCK_F_STREAM (0)] stream socket type is supported.
\item[VIRTIO_VSOCK_F_SEQPACKET (1)] seqpacket socket type is supported.
+\item[VIRTIO_VSOCK_F_SIZE_MAX (2)] Maximum size of the packet payload is in +ÂÂÂÂÂ \field{data_max_size}. If offered by the device, device advises driver +ÂÂÂÂÂ about the value of its maximum payload size. If negotiated, the driver uses
+ÂÂÂÂÂ \field{data_max_size} as the maximum packet payload size value.

\subsection{Device configuration layout}\label{sec:Device Types / Socket Device / Device configuration layout} @@ -32,6 +36,7 @@ \subsection{Device configuration layout}\label{sec:Device Types / Socket Device
struct virtio_vsock_config {
ÂÂÂÂÂ le64 guest_cid;
+ÂÂÂÂÂ le32 data_max_size;

@@ -57,6 +62,28 @@ \subsection{Device configuration layout}\label{sec:Device Types / Socket Device

+The following driver-read-only field, \field{data_max_size} only exists if +VIRTIO_VSOCK_F_SIZE_MAX is set. This field specifies the maximum packet payload
+size for the driver to use.
+\devicenormative{\subsubsection}{Device configuration layout}{Device Types / Socket Device / Device configuration layout}
+The device MUST NOT change the value exposed through \field{data_max_size}.
+The device MUST forward transmitted packets of up to \field{data_max_size} +(plus header length) size after VIRTIO_VSOCK_F_SIZE_MAX has been successfully

Sorry, I thought we were going to delete this sentence like we did the
one above, maybe I didn't get the meaning right though.

What do you mean here by "forward"?


Sorry for the confusion. Now I got super confused as well :D. So actually
"MUST NOT pass received packets" was about the packets received from the driver, and "MUST forward transmitted packets" about the packets transmitted from the backend to the driver. The "receive/transmit" terminology was from the device perspective here. So your initial comment applied to this sentence actually, not the initial one, but I forgot then that I wrote this section from the device perspective. Both sentences are already implied by their corresponding ones from the driver
section, so I think I can delete both actually.
Thanks for raising this!


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