Re: [PATCH v5 04/15] x86/mtrr: support setting MTRR state for software defined MTRRs

[Juergen Gross]

On 03.04.23 04:25, Huang, Kai wrote:
> On Sat, 2023-04-01 at 08:36 +0200, 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.
> > + */
> > +void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
> > +			  mtrr_type def_type)
> > +{
> > +	unsigned int i;
> > +
> > +	/* Only allowed to be called once before mtrr_bp_init(). */
> > +	if (WARN_ON(mtrr_state_set))
> > +		return;
> > +
> > +	/* Only allowed when running virtualized. */
> > +	if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
> > +		return;
> > +
> > +	/*
> > +	 * Only allowed for special virtualization cases:
> > +	 * - when running as SEV-SNP guest
> > +	 * - when running as Hyper-V isolated guest
> > +	 * - when running as Xen PV guest
> > +	 * - when running as TSX guest
> 			     ^
> 			     TDX guest

Thanks.

> > +	 */
> > +	if (!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))
> > +		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 1beb38f7a7a3..1c19d67ddab3 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 (!generic_mtrrs && mtrr_state.enabled) {
> > +		/* Software overwrite of MTRR state, only for generic case. */
> 							      ^
> 							      !generic case?

No. This test just verifies that the (visible) MTRR feature is switched off,
as there are no ways to modify any MTRR registers in the overwrite case.

I can make this more obvious in a comment.


Juergen

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