linux-kernel - Re: [PATCH 15/14] x86/gds: Lock GDS mitigation when keylocker feature is present

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Date: Fri, 19 Apr 2024 13:19:18 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Daniel Sneddon <daniel.sneddon@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, dan.j.williams@...el.com,
	bernie.keany@...el.com, charishma1.gairuboyina@...el.com,
	chang.seok.bae@...el.com, Josh Poimboeuf <jpoimboe@...nel.org>,
	antonio.gomez.iglesias@...ux.intel.com
Subject: Re: [PATCH 15/14] x86/gds: Lock GDS mitigation when keylocker
 feature is present

On Fri, Apr 19, 2024 at 11:03:28AM -0700, Daniel Sneddon wrote:
> On 4/19/24 10:47, Pawan Gupta wrote:
> > +	u64 gds_lock = 0;
> >  
> >  	switch (gds_mitigation) {
> >  	case GDS_MITIGATION_OFF:
> > @@ -769,6 +769,8 @@ void update_gds_msr(void)
> >  		 * the same state. Make sure the mitigation is enabled on all
> >  		 * CPUs.
> >  		 */
> > +		gds_lock = GDS_MITG_LOCKED;
> Can't we just drop the new gds_lock var and set mcu_ctrl |= GDS_MITG_LOCKED here?

Unfortunately no, because ...

> > +		fallthrough;
> >  	case GDS_MITIGATION_FULL:
> >  		rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);

.. mcu_ctrl is read here, it will overwrite any previous value.

> >  		mcu_ctrl &= ~GDS_MITG_DIS;
> > @@ -779,6 +781,7 @@ void update_gds_msr(void)
> >  		return;
> >  	}
> >  
> > +	mcu_ctrl |= gds_lock;
> >  	wrmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);