Re: [PATCH v3 4/7] x86/time: introduce probing logic for the wallclock

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.09.2024 12:58, Roger Pau Monné wrote:
> On Tue, Sep 03, 2024 at 05:32:27PM +0200, Jan Beulich wrote:
>> On 03.09.2024 15:03, Roger Pau Monne wrote:
>>> @@ -1329,28 +1338,13 @@ static bool cmos_probe(struct rtc_time *rtc_p, bool 
>>> cmos_rtc_probe)
>>>      return false;
>>>  }
>>>  
>>> -static unsigned long get_cmos_time(void)
>>> +
>>> +static unsigned long cmos_read(void)
>>>  {
>>> -    unsigned long res;
>>>      struct rtc_time rtc;
>>> -    static bool __read_mostly cmos_rtc_probe;
>>> -    boolean_param("cmos-rtc-probe", cmos_rtc_probe);
>>> +    bool success = __get_cmos_time(&rtc);
>>>  
>>> -    if ( efi_enabled(EFI_RS) )
>>> -    {
>>> -        res = efi_get_time();
>>> -        if ( res )
>>> -            return res;
>>> -    }
>>> -
>>> -    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 )
>>> -        panic("System with no CMOS RTC advertised must be booted from EFI"
>>> -              " (or with command line option \"cmos-rtc-probe\")\n");
>>> -
>>> -    if ( !cmos_probe(&rtc, cmos_rtc_probe) )
>>> -        panic("No CMOS RTC found - system must be booted from EFI\n");
>>> +    ASSERT(success);
>>
>> I'm not convinced of this assertion: It's either too much (compared to
>> what we had so far) or not enough, considering the behavior ...
>>
>>>      return mktime(rtc.year, rtc.mon, rtc.day, rtc.hour, rtc.min, rtc.sec);
>>>  }
>>
>> ... with a release build.
> 
> My reasoning was that on a debug build we want to spot any such
> issues (as it's likely a symptom the RTC is misbehaving?) but on a release
> build we should rather return an incorrect wallclock time rather than
> panicking.  I can remove the ASSERT and local variable altogether if
> you prefer.

I would prefer that, yes, but I also won't insist.

>>> @@ -1533,12 +1527,82 @@ void rtc_guest_write(unsigned int port, unsigned 
>>> int data)
>>>      }
>>>  }
>>>  
>>> -static unsigned long get_wallclock_time(void)
>>> +static enum {
>>> +    WALLCLOCK_UNSET,
>>> +    WALLCLOCK_XEN,
>>> +    WALLCLOCK_CMOS,
>>> +    WALLCLOCK_EFI,
>>> +} wallclock_source __ro_after_init;
>>> +
>>> +static const char *wallclock_type_to_string(void)
>>
>> __init ?
>>
>>>  {
>>> +    switch ( wallclock_source )
>>> +    {
>>> +    case WALLCLOCK_XEN:
>>> +        return "XEN";
>>> +
>>> +    case WALLCLOCK_CMOS:
>>> +        return "CMOS RTC";
>>> +
>>> +    case WALLCLOCK_EFI:
>>> +        return "EFI";
>>> +
>>> +    case WALLCLOCK_UNSET:
>>> +        return "UNSET";
>>> +    }
>>> +
>>> +    ASSERT_UNREACHABLE();
>>> +    return "";
>>> +}
>>> +
>>> +static void __init probe_wallclock(void)
>>> +{
>>> +    ASSERT(wallclock_source == WALLCLOCK_UNSET);
>>> +
>>>      if ( xen_guest )
>>> +    {
>>> +        wallclock_source = WALLCLOCK_XEN;
>>> +        return;
>>> +    }
>>> +    if ( efi_enabled(EFI_RS) && efi_get_time() )
>>> +    {
>>> +        wallclock_source = WALLCLOCK_EFI;
>>> +        return;
>>> +    }
>>> +    if ( cmos_probe() )
>>> +    {
>>> +        wallclock_source = WALLCLOCK_CMOS;
>>> +        return;
>>> +    }
>>> +
>>> +    panic("No usable wallclock found, probed:%s%s%s\n%s",
>>> +          !cmos_rtc_probe && !efi_enabled(EFI_RS) ? " None" : "",
>>> +          cmos_rtc_probe ? " CMOS" : "",
>>> +          efi_enabled(EFI_RS) ? " EFI" : "",
>>> +          !cmos_rtc_probe ? "Try with command line option 
>>> \"cmos-rtc-probe\"\n"
>>> +           : !efi_enabled(EFI_RS) ? "System must be booted from EFI\n" : 
>>> "");
>>
>> This last argument is sufficiently complex that I think it is pretty
>> important for the question marks and colons to respectively align with
>> one another, even if this may mean one or two more lines of code.
> 
> I had it that way originally, but then it seemed the extra
> indentation made it less readable.  Will see how can I adjust it, my
> preference would be for:
> 
>     panic("No usable wallclock found, probed:%s%s%s\n%s",
>           !cmos_rtc_probe && !efi_enabled(EFI_RS) ? " None" : "",
>           cmos_rtc_probe ? " CMOS" : "",
>           efi_enabled(EFI_RS) ? " EFI" : "",
>           !cmos_rtc_probe ? "Try with command line option 
> \"cmos-rtc-probe\"\n"
>                           : !efi_enabled(EFI_RS) ? "System must be booted 
> from EFI\n"
>                                                  : "");
> 
> But that exceeds the 80 columns limit.

Right, formally the above would be my preference, too. Here two shorter-
lines alternatives:

    panic("No usable wallclock found, probed:%s%s%s\n%s",
          !cmos_rtc_probe && !efi_enabled(EFI_RS) ? " None" : "",
          cmos_rtc_probe ? " CMOS" : "",
          efi_enabled(EFI_RS) ? " EFI" : "",
          !cmos_rtc_probe
          ? "Try with command line option \"cmos-rtc-probe\"\n"
          : !efi_enabled(EFI_RS) ? "System must be booted from EFI\n"
                                 : "");

    panic("No usable wallclock found, probed:%s%s%s\n%s",
          !cmos_rtc_probe && !efi_enabled(EFI_RS) ? " None" : "",
          cmos_rtc_probe ? " CMOS" : "",
          efi_enabled(EFI_RS) ? " EFI" : "",
          !cmos_rtc_probe
              ? "Try with command line option \"cmos-rtc-probe\"\n"
              : !efi_enabled(EFI_RS)
                  ? "System must be booted from EFI\n"
                  : "");

Either of these or anything more or less similar will do imo, just as
long as the ? vs : alignment is there.

One thing I notice only now: The trailing %s will be a little odd if
the "" variant is used in the last argument. That'll produce "(XEN) "
with nothing following in the log. Which usually is a sign of some
strange breakage.

Jan