linux-kernel - [PATCH] drivers: hv: log when enabling crash_kexec_post

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Message-Id: <20220215013735.358327-1-stephen.s.brennan@oracle.com>
Date:   Mon, 14 Feb 2022 17:37:35 -0800
From:   Stephen Brennan <stephen.s.brennan@...cle.com>
To:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>
Cc:     Stephen Brennan <stephen.s.brennan@...cle.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] drivers: hv: log when enabling crash_kexec_post_notifiers

Recently I went down a rabbit hole looking at a race condition in
panic() on a Hyper-V guest. I assumed, since it was missing from the
command line, that crash_kexec_post_notifiers was disabled. Only after
a rather long reproduction and analysis process did I learn that Hyper-V
actually enables this setting unconditionally.

Users and debuggers alike would like to know when these things happen. I
think it would be good to print a message to the kernel log when this
happens, so that a grep for "crash_kexec_post_notifiers" shows relevant
results.

Signed-off-by: Stephen Brennan <stephen.s.brennan@...cle.com>
---
 drivers/hv/hv_common.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
index 181d16bbf49d..c1dd21d0d7ef 100644
--- a/drivers/hv/hv_common.c
+++ b/drivers/hv/hv_common.c
@@ -79,8 +79,10 @@ int __init hv_common_init(void)
 	 * calling crash enlightment interface before running kdump
 	 * kernel.
 	 */
-	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE)
+	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE) {
 		crash_kexec_post_notifiers = true;
+		pr_info("Hyper-V: enabling crash_kexec_post_notifiers\n");
+	}
 
 	/*
 	 * Allocate the per-CPU state for the hypercall input arg.
-- 
2.30.2