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hv_utils: Add comment about max VMbus packet size in VSS driver

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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 <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1644423070-75125-1-git-send-email-mikelley@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
This commit is contained in:
Michael Kelley 2022-02-09 08:11:10 -08:00 • committed by Wei Liu
parent 4ee5245871
commit 6de74d1069
2 changed files with 16 additions and 2 deletions
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7
drivers/hv/hv_snapshot.c
View file

@ -31,6 +31,9 @@ static const int fw_versions[] = {
UTIL_FW_VERSION 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 * Timeout values are based on expecations from host
*/ */
@ -298,7 +301,7 @@ void hv_vss_onchannelcallback(void *context)
if (vss_transaction.state > HVUTIL_READY) if (vss_transaction.state > HVUTIL_READY)
return; 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"); pr_err_ratelimited("VSS request received. Could not read into recv buf\n");
return; return;
} }
@ -375,7 +378,7 @@ hv_vss_init(struct hv_util_service *srv)
} }
recv_buffer = srv->recv_buffer; recv_buffer = srv->recv_buffer;
vss_transaction.recv_channel = srv->channel; 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 * When this driver loads, the user level daemon that

11
include/uapi/linux/hyperv.h
View file

@ -90,6 +90,17 @@ struct hv_vss_check_dm_info {
__u32 flags; __u32 flags;
} __attribute__((packed)); } __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 { struct hv_vss_msg {
union { union {
struct hv_vss_hdr vss_hdr; struct hv_vss_hdr vss_hdr;