From 6de74d1069b821e96460d0fc2edfc35785db04fb Mon Sep 17 00:00:00 2001 From: Michael Kelley Date: Wed, 9 Feb 2022 08:11:10 -0800 Subject: [PATCH] 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 Link: https://lore.kernel.org/r/1644423070-75125-1-git-send-email-mikelley@microsoft.com Signed-off-by: Wei Liu --- 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 6018b9d1b1fb..0d2184be1691 100644 --- a/drivers/hv/hv_snapshot.c +++ b/drivers/hv/hv_snapshot.c @@ -31,6 +31,9 @@ static const int fw_versions[] = { 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 @@ hv_vss_init(struct hv_util_service *srv) } 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 daf82a230c0e..aaa502a7bff4 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;