Date:   Sun, 21 Aug 2022 14:25:59 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 01/10] x86/mtrr: fix MTRR fixup on APs

On Sat, Aug 20, 2022 at 11:25:24AM +0200, Juergen Gross wrote:
> When booting or resuming the system MTRR state is saved on the boot
> processor and then this state is loaded into MTRRs of all other cpus.

s/cpu/CPU/g

Pls check all commit messages.

> During update of the MTRRs the MTRR mechanism needs to be disabled by
> writing the related MSR. The old contents of this MSR are saved in a
> set of static variables and later those static variables are used to
> restore the MSR.
> 
> In case the MSR contents need to be modified on a cpu due to the MSR
> not having been initialized properly by the BIOS, the related update
> function is modifying the static variables accordingly.
> 
> Unfortunately the MTRR state update is usually running on all cpus
> at the same time, so using just one set of static variables for all
> cpus is racy in case the MSR contents differ across cpus.
> 
> Fix that by using percpu variables for saving the MSR contents.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
> I thought adding a "Fixes:" tag for the kernel's initial git commit
> would maybe be entertaining, but without being really helpful.
> The percpu variables were preferred over on-stack ones in order to
> avoid more code churn in followup patches decoupling PAT from MTRR
> support.

So if that thing has been broken for so long and no one noticed, we
could just as well not backport to stable at all...

> V2:
> - new patch
> ---
>  arch/x86/kernel/cpu/mtrr/generic.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
> index 558108296f3c..3d185fcf08ca 100644
> --- a/arch/x86/kernel/cpu/mtrr/generic.c
> +++ b/arch/x86/kernel/cpu/mtrr/generic.c
> @@ -679,7 +679,8 @@ static bool set_mtrr_var_ranges(unsigned int index, struct mtrr_var_range *vr)
>  	return changed;
>  }
>  
> -static u32 deftype_lo, deftype_hi;
> +static DEFINE_PER_CPU(u32, deftype_lo);
> +static DEFINE_PER_CPU(u32, deftype_hi);

My APM says that the high 32 bits of the MTRRdefType MSR are reserved
and MBZ. So you can drop the _hi thing and use 0 everywhere. Or rather a
dummy = 0 var because of that damn rdmsr() macro.

Or simply use a

u64 deftype;

use the rdmsrl/wrmsrl() variants and split it when passing to
umtrr_wrmsr() because that thing wants 2 32s.

>  /**
>   * set_mtrr_state - Set the MTRR state for this CPU.
> @@ -691,6 +692,7 @@ static unsigned long set_mtrr_state(void)
>  {
>  	unsigned long change_mask = 0;
>  	unsigned int i;
> +	u32 *lo = this_cpu_ptr(&deftype_lo);

The tip-tree preferred ordering of variable declarations at the
beginning of a function is reverse fir tree order::

	struct long_struct_name *descriptive_name;
	unsigned long foo, bar;
	unsigned int tmp;
	int ret;

The above is faster to parse than the reverse ordering::

	int ret;
	unsigned int tmp;
	unsigned long foo, bar;
	struct long_struct_name *descriptive_name;

And even more so than random ordering::

	unsigned long foo, bar;
	int ret;
	struct long_struct_name *descriptive_name;
	unsigned int tmp;

Please check all your patches.

>  	for (i = 0; i < num_var_ranges; i++) {
>  		if (set_mtrr_var_ranges(i, &mtrr_state.var_ranges[i]))
> @@ -704,10 +706,10 @@ static unsigned long set_mtrr_state(void)
>  	 * Set_mtrr_restore restores the old value of MTRRdefType,
>  	 * so to set it we fiddle with the saved value:
>  	 */
> -	if ((deftype_lo & 0xff) != mtrr_state.def_type
> -	    || ((deftype_lo & 0xc00) >> 10) != mtrr_state.enabled) {
> +	if ((*lo & 0xff) != mtrr_state.def_type
> +	    || ((*lo & 0xc00) >> 10) != mtrr_state.enabled) {
>  
> -		deftype_lo = (deftype_lo & ~0xcff) | mtrr_state.def_type |
> +		*lo = (*lo & ~0xcff) | mtrr_state.def_type |
>  			     (mtrr_state.enabled << 10);
>  		change_mask |= MTRR_CHANGE_MASK_DEFTYPE;
>  	}
> @@ -729,6 +731,8 @@ static DEFINE_RAW_SPINLOCK(set_atomicity_lock);
>  static void prepare_set(void) __acquires(set_atomicity_lock)
>  {
>  	unsigned long cr0;
> +	u32 *lo = this_cpu_ptr(&deftype_lo);
> +	u32 *hi = this_cpu_ptr(&deftype_hi);

You don't need the pointers here - this_cpu_read() should be enough.

>  	/*
>  	 * Note that this is not ideal
> @@ -763,10 +767,10 @@ static void prepare_set(void) __acquires(set_atomicity_lock)
>  	flush_tlb_local();
>  
>  	/* Save MTRR state */
> -	rdmsr(MSR_MTRRdefType, deftype_lo, deftype_hi);
> +	rdmsr(MSR_MTRRdefType, *lo, *hi);
>  
>  	/* Disable MTRRs, and set the default type to uncached */
> -	mtrr_wrmsr(MSR_MTRRdefType, deftype_lo & ~0xcff, deftype_hi);
> +	mtrr_wrmsr(MSR_MTRRdefType, *lo & ~0xcff, *hi);
>  
>  	/* Again, only flush caches if we have to. */
>  	if (!static_cpu_has(X86_FEATURE_SELFSNOOP))
> @@ -775,12 +779,15 @@ static void prepare_set(void) __acquires(set_atomicity_lock)
>  
>  static void post_set(void) __releases(set_atomicity_lock)
>  {
> +	u32 *lo = this_cpu_ptr(&deftype_lo);
> +	u32 *hi = this_cpu_ptr(&deftype_hi);

Ditto.

Thx.

-- 
Regards/Gruss,
    Boris.

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