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

From: Juergen Gross <jgross@suse.com>
To: Borislav Petkov <bp@alien8.de>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v4 03/12] x86/mtrr: support setting MTRR state for software defined MTRRs
Date: Tue, 21 Mar 2023 07:00:58 +0100	[thread overview]
Message-ID: <01ec6ce2-7639-2e4f-3433-c77c29fa7984@suse.com> (raw)
In-Reply-To: <20230320190543.GFZBiuh2lImP8LEW3D@fat_crate.local>

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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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