2022-02-09 18:04:35

by Michael Kelley (LINUX)

Subject: [PATCH 1/1] hv_utils: Add comment about max VMbus packet size in VSS driver

The VSS driver allocates a VMbus receive buffer significantly
larger than sizeof(hv_vss_msg), with no explanation. To help
prevent future mistakes, add a #define and comment about why
this is done.

No functional change.

Signed-off-by: Michael Kelley <[email protected]>
---
drivers/hv/hv_snapshot.c | 7 +++++--
include/uapi/linux/hyperv.h | 11 +++++++++++
2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/hv_snapshot.c b/drivers/hv/hv_snapshot.c
index 6018b9d..0d2184b 100644
--- a/drivers/hv/hv_snapshot.c
+++ b/drivers/hv/hv_snapshot.c
@@ -31,6 +31,9 @@
UTIL_FW_VERSION
};

+/* See comment with struct hv_vss_msg regarding the max VMbus packet size */
+#define VSS_MAX_PKT_SIZE (HV_HYP_PAGE_SIZE * 2)
+
/*
* Timeout values are based on expecations from host
*/
@@ -298,7 +301,7 @@ void hv_vss_onchannelcallback(void *context)
if (vss_transaction.state > HVUTIL_READY)
return;

- if (vmbus_recvpacket(channel, recv_buffer, HV_HYP_PAGE_SIZE * 2, &recvlen, &requestid)) {
+ if (vmbus_recvpacket(channel, recv_buffer, VSS_MAX_PKT_SIZE, &recvlen, &requestid)) {
pr_err_ratelimited("VSS request received. Could not read into recv buf\n");
return;
}
@@ -375,7 +378,7 @@ static void vss_on_reset(void)
}
recv_buffer = srv->recv_buffer;
vss_transaction.recv_channel = srv->channel;
- vss_transaction.recv_channel->max_pkt_size = HV_HYP_PAGE_SIZE * 2;
+ vss_transaction.recv_channel->max_pkt_size = VSS_MAX_PKT_SIZE;

/*
* When this driver loads, the user level daemon that
diff --git a/include/uapi/linux/hyperv.h b/include/uapi/linux/hyperv.h
index daf82a2..aaa502a 100644
--- a/include/uapi/linux/hyperv.h
+++ b/include/uapi/linux/hyperv.h
@@ -90,6 +90,17 @@ struct hv_vss_check_dm_info {
__u32 flags;
} __attribute__((packed));

+/*
+ * struct hv_vss_msg encodes the fields that the Linux VSS
+ * driver accesses. However, FREEZE messages from Hyper-V contain
+ * additional LUN information that Linux doesn't use and are not
+ * represented in struct hv_vss_msg. A received FREEZE message may
+ * be as large as 6,260 bytes, so the driver must allocate at least
+ * that much space, not sizeof(struct hv_vss_msg). Other messages
+ * such as AUTO_RECOVER may be as large as 12,500 bytes. However,
+ * because the Linux VSS driver responds that it doesn't support
+ * auto-recovery, it should not receive such messages.
+ */
struct hv_vss_msg {
union {
struct hv_vss_hdr vss_hdr;
--
1.8.3.1

2022-02-18 14:59:21

by Wei Liu

Subject: Re: [PATCH 1/1] hv_utils: Add comment about max VMbus packet size in VSS driver

On Wed, Feb 09, 2022 at 08:11:10AM -0800, Michael Kelley wrote:
> The VSS driver allocates a VMbus receive buffer significantly
> larger than sizeof(hv_vss_msg), with no explanation. To help
> prevent future mistakes, add a #define and comment about why
> this is done.
>
> No functional change.
>
> Signed-off-by: Michael Kelley <[email protected]>

Applied to hyperv-next. Thanks.