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

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Date:   Mon, 3 Apr 2023 09:10:10 +0200
From:   Juergen Gross <jgross@...e.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "mikelley@...rosoft.com" <mikelley@...rosoft.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v5 04/15] x86/mtrr: support setting MTRR state for
 software defined MTRRs

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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