Re: [PATCH] x86/spec: fix BRANCH_HARDEN option to only be set when build-enabled

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/02/2024 11:11 am, Roger Pau Monné wrote:
> On Fri, Feb 23, 2024 at 10:26:15AM +0000, Andrew Cooper wrote:
>> On 23/02/2024 10:17 am, Roger Pau Monné wrote:
>>> On Fri, Feb 23, 2024 at 09:46:27AM +0000, Andrew Cooper wrote:
>>>> On 23/02/2024 9:42 am, Roger Pau Monne wrote:
>>>>> The current logic to handle the BRANCH_HARDEN option will report it as 
>>>>> enabled
>>>>> even when build-time disabled. Fix this by only allowing the option to be 
>>>>> set
>>>>> when support for it is built into Xen.
>>>>>
>>>>> Fixes: 2d6f36daa086 ('x86/nospec: Introduce 
>>>>> CONFIG_SPECULATIVE_HARDEN_BRANCH')
>>>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>>> ---
>>>>>  xen/arch/x86/spec_ctrl.c | 6 ++++--
>>>>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
>>>>> index 421fe3f640df..e634c6b559b4 100644
>>>>> --- a/xen/arch/x86/spec_ctrl.c
>>>>> +++ b/xen/arch/x86/spec_ctrl.c
>>>>> @@ -50,7 +50,8 @@ static int8_t __initdata opt_psfd = -1;
>>>>>  int8_t __ro_after_init opt_ibpb_ctxt_switch = -1;
>>>>>  int8_t __read_mostly opt_eager_fpu = -1;
>>>>>  int8_t __read_mostly opt_l1d_flush = -1;
>>>>> -static bool __initdata opt_branch_harden = true;
>>>>> +static bool __initdata opt_branch_harden =
>>>>> +    IS_ENABLED(CONFIG_SPECULATIVE_HARDEN_BRANCH);
>>>>>  
>>>>>  bool __initdata bsp_delay_spec_ctrl;
>>>>>  uint8_t __read_mostly default_xen_spec_ctrl;
>>>>> @@ -267,7 +268,8 @@ static int __init cf_check parse_spec_ctrl(const char 
>>>>> *s)
>>>>>              opt_eager_fpu = val;
>>>>>          else if ( (val = parse_boolean("l1d-flush", s, ss)) >= 0 )
>>>>>              opt_l1d_flush = val;
>>>>> -        else if ( (val = parse_boolean("branch-harden", s, ss)) >= 0 )
>>>>> +        else if ( IS_ENABLED(CONFIG_SPECULATIVE_HARDEN_BRANCH) &&
>>>>> +                  (val = parse_boolean("branch-harden", s, ss)) >= 0 )
>>>>>              opt_branch_harden = val;
>>>> Yeah, we should definitely fix this, but could we use no_config_param()
>>>> here for the compiled-out case ?
>>>>
>>>> See cet= for an example.  If we're going to ignore what the user asks,
>>>> we should tell them why.
>>> Maybe I'm missing something: I've looked into using no_config_param(),
>>> but there's no difference really, because cmdline_parse() is called
>>> before the console is initialized, so those messages seem to be
>>> lost.
>> Look at `xl dmesg` rather than the console.  They also do appear on vga
>> in some configurations.
> Oh, my internal buffer was too small on those also got truncated, had
> to bump it.

Yeah - the default buffer size in Xen is too small.  It has been for
more than a decade, which is how long the adjustment in XenServer has
been around.

>
>> There's a separate todo to get these out in a slightly nicer way, but
>> they at least exist in logs.
> I've created:
>
> https://gitlab.com/xen-project/xen/-/issues/184

Thanks.

~Andrew