[PATCH v2 2/2] x86/time: prefer CMOS over EFI_GET_TIME

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The EFI_GET_TIME implementation is well known to be broken for many firmware
implementations, for Xen the result on such implementations are:

----[ Xen-4.19-unstable  x86_64  debug=y  Tainted:   C    ]----
CPU:    0
RIP:    e008:[<0000000062ccfa70>] 0000000062ccfa70
[...]
Xen call trace:
   [<0000000062ccfa70>] R 0000000062ccfa70
   [<00000000732e9a3f>] S 00000000732e9a3f
   [<ffff82d04034f34f>] F arch/x86/time.c#get_cmos_time+0x1b3/0x26e
   [<ffff82d04045926f>] F init_xen_time+0x28/0xa4
   [<ffff82d040454bc4>] F __start_xen+0x1ee7/0x2578
   [<ffff82d040203334>] F __high_start+0x94/0xa0

Pagetable walk from 0000000062ccfa70:
 L4[0x000] = 000000207ef1c063 ffffffffffffffff
 L3[0x001] = 000000005d6c0063 ffffffffffffffff
 L2[0x116] = 8000000062c001e3 ffffffffffffffff (PSE)

****************************************
Panic on CPU 0:
FATAL PAGE FAULT
[error_code=0011]
Faulting linear address: 0000000062ccfa70
****************************************

Swap the preference to default to CMOS first, and EFI later, in an attempt to
use EFI_GET_TIME as a last resort option only.  Note that Linux for example
doesn't allow calling the get_time method, and instead provides a dummy handler
that unconditionally returns EFI_UNSUPPORTED on x86-64.

Such change in the preferences requires some re-arranging of the function
logic, so that panic messages with workaround suggestions are suitably printed.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/time.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 272ca2468ea6..0eee954809a9 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1305,24 +1305,36 @@ static unsigned long get_cmos_time(void)
     static bool __read_mostly cmos_rtc_probe;
     boolean_param("cmos-rtc-probe", cmos_rtc_probe);
 
+    if ( likely(!(acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_CMOS_RTC)) )
+        cmos_rtc_probe = false;
+
+    if ( (!(acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_CMOS_RTC) ||
+          cmos_rtc_probe) && read_cmos_time(&rtc, cmos_rtc_probe) )
+        return mktime(rtc.year, rtc.mon, rtc.day, rtc.hour, rtc.min, rtc.sec);
+
     if ( efi_enabled(EFI_RS) )
     {
         unsigned long res = efi_get_time();
 
         if ( res )
             return res;
+
+        panic("Broken EFI_GET_TIME %s\n",
+              !cmos_rtc_probe ? "try booting with \"cmos-rtc-probe\" option"
+                              : "and no CMOS RTC found");
     }
 
-    if ( likely(!(acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_CMOS_RTC)) )
-        cmos_rtc_probe = false;
-    else if ( system_state < SYS_STATE_smp_boot && !cmos_rtc_probe )
+    /*
+     * No viable clock source found.  Attempt to provide some guidance to the
+     * user about possible workarounds to boot Xen on the system.
+     */
+    ASSERT(system_state < SYS_STATE_smp_boot);
+
+    if ( !cmos_rtc_probe )
         panic("System with no CMOS RTC advertised must be booted from EFI"
               " (or with command line option \"cmos-rtc-probe\")\n");
 
-    if ( unlikely(!read_cmos_time(&rtc, cmos_rtc_probe)) )
-        panic("No CMOS RTC found - system must be booted from EFI\n");
-
-    return mktime(rtc.year, rtc.mon, rtc.day, rtc.hour, rtc.min, rtc.sec);
+    panic("No CMOS RTC found - system must be booted from EFI\n");
 }
 
 static unsigned int __ro_after_init cmos_alias_mask;
-- 
2.46.0