Re: [PATCH 15/22] x86/traps: Introduce opt_fred

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.08.2025 21:16, Andrew Cooper wrote:
> On 14/08/2025 2:30 pm, Jan Beulich wrote:
>> On 08.08.2025 22:23, Andrew Cooper wrote:
>>> ... disabled by default.  There is a lot of work before FRED can be enabled 
>>> by
>>> default.
>>>
>>> One part of FRED, the LKGS (Load Kernel GS) instruction, is enumerated
>>> separately but is mandatory as FRED disallows the SWAPGS instruction.
>>> Therefore, both CPUID bits must be checked.
>> See my (further) reply to patch 13 - I think FRED simply ought to depend on
>> LKGS.
>>
>>> @@ -20,6 +22,9 @@ unsigned int __ro_after_init ler_msr;
>>>  static bool __initdata opt_ler;
>>>  boolean_param("ler", opt_ler);
>>>  
>>> +int8_t __ro_after_init opt_fred = 0; /* -1 when supported. */
>> I'm a little puzzled by the comment? DYM "once default-enabled"?
> 
> Well, I have this temporary patch
> https://gitlab.com/xen-project/hardware/xen-staging/-/commit/70ef6a1178a411a29b7b1745a1112e267ffb6245
> that will turn into a real patch when we enable FRED by default.
> 
> As much as anything else, it was just a TODO.
> 
> 
>>  Then ...
>>
>>> @@ -305,6 +310,32 @@ void __init traps_init(void)
>>>      /* Replace early pagefault with real pagefault handler. */
>>>      _update_gate_addr_lower(&bsp_idt[X86_EXC_PF], entry_PF);
>>>  
>>> +    if ( !cpu_has_fred || !cpu_has_lkgs )
>>> +    {
>>> +        if ( opt_fred )
>> ... this won't work anymore once the initializer is changed.
> 
> Hmm yes.  That wants to be an == 1 check.  Fixed.
> 
>>
>>> +            printk(XENLOG_WARNING "FRED not available, ignoring\n");
>>> +        opt_fred = false;
>> Better use 0 here?
>>
>>> +    }
>>> +
>>> +    if ( opt_fred == -1 )
>>> +        opt_fred = !pv_shim;
>> Imo it would be better to have the initializer be -1 right away, and comment
>> out the "!pv_shim" here, until we mean it to be default-enabled.
> 
> It cannot be -1, or Xen will fail spectacularly on any FRED capable
> hardware.  Setting to -1 is the point at which FRED becomes security
> supported.

I guess I'm not following: If it was -1, and if the code here was

    if ( opt_fred < 0 )
        opt_fred = 0 /* !pv_shim */;

why would things "fail spectacularly" unless someone passed "fred" on
the command line?

Jan