Re: [PATCH v4 03/12] x86/mtrr: support setting MTRR state for software defined MTRRs

[Juergen Gross]

On 20.03.23 20:05, Borislav Petkov wrote:
> On Mon, Mar 06, 2023 at 05:34:16PM +0100, Juergen Gross wrote:
> > +/**
> > + * mtrr_overwrite_state - set static MTRR state
> > + *
> > + * Used to set MTRR state via different means (e.g. with data obtained from
> > + * a hypervisor).
> > + * Is allowed only for special cases when running virtualized. Must be called
> > + * from the x86_init.hyper.init_platform() hook. X86_FEATURE_MTRR must be off.
> > + */
> > +void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
> > +			  mtrr_type def_type)
> > +{
> > +	unsigned int i;
> > +
> > +	if (WARN_ON(mtrr_state_set ||
> > +		    hypervisor_is_type(X86_HYPER_NATIVE) ||
>
> Why that check?

I guess you are asking because the next test seems to catch the same case?

I think it doesn't, e.g. for the case of unknown hypervisors (which shows that
X86_HYPER_NATIVE in theory should be named X86_HYPER_NATIVE_OR_UNKNOWN, or it
should be split into X86_HYPER_NATIVE and X86_HYPER_UNKNOWN).

> > +		    !cpu_feature_enabled(X86_FEATURE_HYPERVISOR) ||
> > +		    (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP) &&
> > +		     !hv_is_isolation_supported() &&
> > +		     !cpu_feature_enabled(X86_FEATURE_XENPV) &&
> > +		     !cpu_feature_enabled(X86_FEATURE_TDX_GUEST))))
>
> This is unparseable. Please split it into separate checks:
>
> 	if (WARN_ON(mtrr_state_set))
> 		return;
>
> 	if (WARN_ON(!cpu_feature_enabled(X86_FEATURE_HYPERVISOR)))
> 		return;
>
> 	...
>
> and add comments above each one why we're testing this.

Okay.

> > +		return;
> > +
> > +	/* Disable MTRR in order to disable MTRR modifications. */
> > +	setup_clear_cpu_cap(X86_FEATURE_MTRR);
> > +
> > +	if (var) {
> > +		if (num_var > MTRR_MAX_VAR_RANGES) {
> > +			pr_warn("Trying to overwrite MTRR state with %u variable entries\n",
> > +				num_var);
> > +			num_var = MTRR_MAX_VAR_RANGES;
> > +		}
> > +		for (i = 0; i < num_var; i++)
> > +			mtrr_state.var_ranges[i] = var[i];
> > +		num_var_ranges = num_var;
> > +	}
> > +
> > +	mtrr_state.def_type = def_type;
> > +	mtrr_state.enabled |= MTRR_STATE_MTRR_ENABLED;
> > +
> > +	mtrr_state_set = 1;
> > +}
> > +
> >   /**
> >    * mtrr_type_lookup - look up memory type in MTRR
> >    *
> > diff --git a/arch/x86/kernel/cpu/mtrr/mtrr.c b/arch/x86/kernel/cpu/mtrr/mtrr.c
> > index 7596ebeab929..5fe62ee0361b 100644
> > --- a/arch/x86/kernel/cpu/mtrr/mtrr.c
> > +++ b/arch/x86/kernel/cpu/mtrr/mtrr.c
> > @@ -666,6 +666,15 @@ void __init mtrr_bp_init(void)
> >   	const char *why = "(not available)";
> >   	unsigned int phys_addr;
> >   
> > +	if (mtrr_state.enabled) {
>
> I'm guessing the proper detection of that weird state should be:
>
> 	/*
> 	 * Check for the software overwrite of MTRR state, only for generic case.
> 	 * See mtrr_overwrite_state().
> 	 */
> 	if (!cpu_feature_enabled(X86_FEATURE_MTRR) &&
> 	    mtrr_state.enabled) {
> 		...

It basically doesn't matter.

The only possibility of mtrr_state.enabled to be set at this point is a
previous call of mtrr_overwrite_state().


Juergen

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