Re: [PATCH v7 2/2] x86/time: prefer CMOS over EFI_GET_TIME

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/09/2024 8:59 am, Roger Pau Monne wrote:
> 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>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

This brings us better-in-line with the rest of the world on the matter.

~Andrew