Drivers: hv: Always do Hyper-V panic notification in hv_kmsg_dump()

hv_kmsg_dump() currently skips the panic notification entirely if it
doesn't get any message bytes to pass to Hyper-V due to an error from
kmsg_dump_get_buffer(). Skipping the notification is undesirable because
it leaves the Hyper-V host uncertain about the state of a panic'ed guest.

Fix this by always doing the panic notification, even if bytes_written
is zero. Also ensure that bytes_written is initialized, which fixes a
kernel test robot warning. The warning is actually bogus because
kmsg_dump_get_buffer() happens to set bytes_written even if it fails, and
in the kernel test robot's CONFIG_PRINTK not set case, hv_kmsg_dump() is
never called. But do the initialization for robustness and to quiet the
static checker.

Fixes: 9c318a1 ("Drivers: hv: move panic report code from vmbus to hv early init code")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/202512172103.OcUspn1Z-lkp@intel.com/
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Roman Kisel <vdso@mailbox.org>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

Author: mhklinux

drivers/hv/hv_common.c

@@ -195,13 +195,15 @@ static void hv_kmsg_dump(struct kmsg_dumper *dumper,
 
 	/*
 	 * Write dump contents to the page. No need to synchronize; panic should
-	 * be single-threaded.
+	 * be single-threaded. Ignore failures from kmsg_dump_get_buffer() since
+	 * panic notification should be done even if there is no message data.
+	 * Don't assume bytes_written is set in case of failure, so initialize it.
 	 */
 	kmsg_dump_rewind(&iter);
-	kmsg_dump_get_buffer(&iter, false, hv_panic_page, HV_HYP_PAGE_SIZE,
+	bytes_written = 0;
+	(void)kmsg_dump_get_buffer(&iter, false, hv_panic_page, HV_HYP_PAGE_SIZE,
 			     &bytes_written);
-	if (!bytes_written)
-		return;
+
 	/*
 	 * P3 to contain the physical address of the panic page & P4 to
 	 * contain the size of the panic data in that page. Rest of the
@@ -210,7 +212,7 @@ static void hv_kmsg_dump(struct kmsg_dumper *dumper,
 	hv_set_msr(HV_MSR_CRASH_P0, 0);
 	hv_set_msr(HV_MSR_CRASH_P1, 0);
 	hv_set_msr(HV_MSR_CRASH_P2, 0);
-	hv_set_msr(HV_MSR_CRASH_P3, virt_to_phys(hv_panic_page));
+	hv_set_msr(HV_MSR_CRASH_P3, bytes_written ? virt_to_phys(hv_panic_page) : 0);
 	hv_set_msr(HV_MSR_CRASH_P4, bytes_written);
 
 	/*