On Wed, Oct 13, 2021 at 03:22:30PM +0100, Jane Malalane wrote:
> @@ -650,6 +651,27 @@ static void early_init_amd(struct cpuinfo_x86 *c)
> if (c->x86_power & BIT(14))
> set_cpu_cap(c, X86_FEATURE_RAPL);
>
> + /*
> + * Zen1 and earlier CPUs don't clear segment base/limits when
> + * loading a NULL selector. This has been designated
> + * X86_BUG_NULL_SEG.
> + *
> + * Zen3 CPUs advertise Null Selector Clears Base in CPUID.
> + * Zen2 CPUs also have this behaviour, but no CPUID bit.
> + *
> + * A hypervisor may sythesize the bit, but may also hide it
> + * for migration safety, so we must not probe for model
> + * specific behaviour when virtualised.
> + */
> + if (c->extended_cpuid_level >= 0x80000021 && cpuid_eax(0x80000021) & BIT(6))
> + nscb = true;
> +
> + if (!cpu_has(c, X86_FEATURE_HYPERVISOR) && !nscb && c->x86 == 0x17)
> + nscb = check_null_seg_clears_base(c);
> +
> + if (!nscb)
> + set_cpu_bug(c, X86_BUG_NULL_SEG);
> +
> #ifdef CONFIG_X86_64
> set_cpu_cap(c, X86_FEATURE_SYSCALL32);
> #else
Can we do something like this?
It is carved out into a separate function which you can simply call from
early_init_amd() and early_init_hygon().
I guess you can put that function in arch/x86/kernel/cpu/common.c or so.
Then, you should put the comments right over the code like I've done
below so that one can follow what's going on with each particular check.
I've also flipped the logic a bit and it is simpler this way.
Totally untested of course.
static void early_probe_null_seg_clearing_base(struct cpuinfo_x86 *c)
{
/*
* A hypervisor may sythesize the bit, but may also hide it
* for migration safety, so do not probe for model-specific
* behaviour when virtualised.
*/
if (cpu_has(c, X86_FEATURE_HYPERVISOR))
return;
/* Zen3 CPUs advertise Null Selector Clears Base in CPUID. */
if (c->extended_cpuid_level >= 0x80000021 && cpuid_eax(0x80000021) & BIT(6))
return;
/* Zen2 CPUs also have this behaviour, but no CPUID bit. */
if (c->x86 == 0x17 && check_null_seg_clears_base(c))
return;
/* All the remaining ones are affected */
set_cpu_bug(c, X86_BUG_NULL_SEG);
}
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 0f8885949e8c..2ca4afb97247 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1395,7 +1395,7 @@ void __init early_cpu_init(void)
> early_identify_cpu(&boot_cpu_data);
> }
>
> -static void detect_null_seg_behavior(struct cpuinfo_x86 *c)
> +bool check_null_seg_clears_base(struct cpuinfo_x86 *c)
> {
> #ifdef CONFIG_X86_64
> /*
> @@ -1418,10 +1418,10 @@ static void detect_null_seg_behavior(struct cpuinfo_x86 *c)
> wrmsrl(MSR_FS_BASE, 1);
> loadsegment(fs, 0);
> rdmsrl(MSR_FS_BASE, tmp);
> - if (tmp != 0)
> - set_cpu_bug(c, X86_BUG_NULL_SEG);
> wrmsrl(MSR_FS_BASE, old_base);
> + return tmp == 0;
> #endif
> + return true;
> }
>
> static void generic_identify(struct cpuinfo_x86 *c)
> @@ -1457,8 +1457,6 @@ static void generic_identify(struct cpuinfo_x86 *c)
>
> get_model_name(c); /* Default name */
>
> - detect_null_seg_behavior(c);
> -
> /*
> * ESPFIX is a strange bug. All real CPUs have it. Paravirt
> * systems that run Linux at CPL > 0 may or may not have the
So this function is called on all x86 CPUs. Are you sure others besides
AMD and Hygon do not have the same issue?
IOW, I wouldn't remove that call here.
But then this is the identify() phase in the boot process and you've
moved it to early_identify() by putting it in the ->c_early_init()
function pointer on AMD and Hygon.
Is there any particular reasoning for this or can that detection remain
in ->c_identify()?
Because if this null seg behavior detection should happen on all
CPUs - and I think it should, because, well, it has been that way
until now - then the vendor specific identification minus what
detect_null_seg_behavior() does should run first and then after
->c_identify() is done, you should do something like:
if (!cpu_has_bug(c, X86_BUG_NULL_SEG)) {
if (!check_null_seg_clears_base(c))
set_cpu_bug(c, X86_BUG_NULL_SEG);
}
so that it still takes place on all CPUs.
I.e., you should split the detection.
I hope I'm making sense ...
Ah, btw, that @c parameter to detect_null_seg_behavior() is unused - you
should remove it in a pre-patch.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
