[PATCH v2 12/12] x86/traps: Fix up invalid PASID

[Peter Zijlstra]

On Fri, Jun 12, 2020 at 05:41:33PM -0700, Fenghua Yu wrote:
> +/*
> + * Apply some heuristics to see if the #GP fault was caused by a thread
> + * that hasn't had the IA32_PASID MSR initialized.  If it looks like that
> + * is the problem, try initializing the IA32_PASID MSR. If the heuristic
> + * guesses incorrectly, take one more #GP fault.

How is that going to help? Aren't we then going to run this same
heuristic again and again and again?

> + */
> +bool __fixup_pasid_exception(void)
> +{
> +	u64 pasid_msr;
> +	unsigned int pasid;
> +
> +	/*
> +	 * This function is called only when this #GP was triggered from user
> +	 * space. So the mm cannot be NULL.
> +	 */
> +	pasid = current->mm->pasid;
> +	/* If the mm doesn't have a valid PASID, then can't help. */
> +	if (invalid_pasid(pasid))
> +		return false;
> +
> +	/*
> +	 * Since IRQ is disabled now, the current task still owns the FPU on

That's just weird and confusing. What you want to say is that you rely
on the exception disabling the interrupt.

> +	 * this CPU and the PASID MSR can be directly accessed.
> +	 *
> +	 * If the MSR has a valid PASID, the #GP must be for some other reason.
> +	 *
> +	 * If rdmsr() is really a performance issue, a TIF_ flag may be
> +	 * added to check if the thread has a valid PASID instead of rdmsr().

I don't understand any of this. Nobody except us writes to this MSR, we
should bloody well know what's in it. What gives?

> +	 */
> +	rdmsrl(MSR_IA32_PASID, pasid_msr);
> +	if (pasid_msr & MSR_IA32_PASID_VALID)
> +		return false;
> +
> +	/* Fix up the MSR if the MSR doesn't have a valid PASID. */
> +	wrmsrl(MSR_IA32_PASID, pasid | MSR_IA32_PASID_VALID);
> +
> +	return true;
> +}
> -- 
> 2.19.1
>