Re: [PATCH] x86: Clear LAM and FRED feature bits

[Xin Li]

On 7/22/2025 9:46 AM, H. Peter Anvin wrote:
> > The following code will work as a generic fix:
> >
> > 	c->x86_capability[i] &= ~DISABLED_MASK(i);
> >
> > And DISABLED_MASK(x) needs to be defined like DISABLED_MASK_BIT_SET(x).
> >
> > Thanks!
> >     Xin
> The easiest thing would be to initialize the setup disabled mask with the DISABLED bitmask instead of zero. This can be done statically; if it isn't already the awk script can produce the disabled bitmask in array form.

Yes, something like:

	void __init init_cpu_cap(void)
	{
		for (i = 0; i < NCAPINTS; i++) {
			cpu_caps_set[i] = REQUIRED_MASK(i);
			cpu_caps_cleared[i] = DISABLED_MASK(i);
		}
	}

And it would be better if it could be done at build time (to avoid
changing Xen which has a dedicated startup code path):

	__u32 cpu_caps_{set,cleared}[NCAPINTS + NBUGINTS] = {
		{REQUIRED,DISABLED}_MASK(i),
	};

And then apply_forced_caps() will do the rest automatically :)