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SubjectRe: [PATCH] x86: introduce a new Linux defined feature flag for PAT support
On Thu, 8 May 2008, Rene Herman wrote:
> However, I'm not sure, but:
>
> > + /* Paranoia check. */
> > + if (!cpu_has_pat) {
> > + printk(KERN_ERR "PAT enabled, but CPU feature cleared\n");
> > + /*
> > + * Panic if this happens on the secondary CPU, and we
> > + * switched to PAT on the boot CPU. We have no way to
> > + * undo PAT.
> > + */
> > + BUG_ON(boot_pat_state);
> > + }
>
> The 'if this happens on the secondary CPU' sounds a bit like this is
> directly checking the secondary CPU flag but cpu_has_pat translates into
> boot_cpu_has(X86_FEATURE_PAT), refers always to the boot cpu.

Right and thats fine because of:

/*
* On SMP, boot_cpu_data holds the common feature set between
* all CPUs; so make sure that we indicate which features are
* common between the CPUs. The first time this routine gets
* executed, c == &boot_cpu_data.
*/
if (c != &boot_cpu_data) {
/* AND the already accumulated flags with these */
for (i = 0 ; i < NCAPINTS ; i++)
boot_cpu_data.x86_capability[i] &= c->x86_capability[i];
}

Thanks,
tglx


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