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