linux-kernel - Re: [patch 12/12] x86/cacheinfo.c: check for block interference CPUs

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Date: Wed, 7 Feb 2024 10:16:29 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org,
	Daniel Bristot de Oliveira <bristot@...nel.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Leonardo Bras <leobras@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 12/12] x86/cacheinfo.c: check for block interference CPUs

On Wed, Feb 07, 2024 at 10:10:41AM -0300, Marcelo Tosatti wrote:
> On Wed, Feb 07, 2024 at 01:41:36PM +0100, Thomas Gleixner wrote:
> > On Tue, Feb 06 2024 at 15:49, Marcelo Tosatti wrote:
> > > @@ -396,6 +397,7 @@ static void amd_l3_disable_index(struct
> > >  	 *  disable index in all 4 subcaches
> > >  	 */
> > >  	for (i = 0; i < 4; i++) {
> > > +		int ret;
> > >  		u32 reg = idx | (i << 20);
> > >  
> > >  		if (!nb->l3_cache.subcaches[i])
> > > @@ -409,6 +411,7 @@ static void amd_l3_disable_index(struct
> > >  		 * is not sufficient.
> > >  		 */
> > >  		ret = wbinvd_on_cpu(cpu);
> > > +		WARN_ON(ret == -EPERM);
> > 
> > What? You create inconsistent state here.
> 
> That should not happen, since we checked for 
> 
> +   idx = block_interf_srcu_read_lock();
> +
> +   if (block_interf_cpu(cpu))
> +           ret = -EPERM;
> 
> Earlier.
> 
> Thus the WARN_ON (hum, can change to BUG_ON...).
> 
> > > -	amd_l3_disable_index(nb, cpu, slot, index);
> > > +	ret = 0;
> > > +	idx = block_interf_srcu_read_lock();
> > > +
> > > +	if (block_interf_cpu(cpu))
> > > +		ret = -EPERM;
> > > +	else
> > > +		amd_l3_disable_index(nb, cpu, slot, index);
> > > +
> > > +	block_interf_srcu_read_unlock(idx);
> > 
> > Again. This is a root only operation.
> > 
> > This whole patch series is just voodoo programming with zero
> > justification for the mess it creates.

BTW, this converted less than 17% callers, so this is why early
feedback is useful.

Thanks!