RE: [PATCH v3 03/12] x86/mtrr: support setting MTRR state for software defined MTRRs

[Michael Kelley (LINUX)]

From: Juergen Gross <jgross@xxxxxxxx> Sent: Thursday, February 23, 2023 1:33 AM
> 
> When running virtualized, MTRR access can be reduced (e.g. in Xen PV
> guests or when running as a SEV-SNP guest under Hyper-V). Typically
> the hypervisor will reset the MTRR feature in CPUID data, resulting
> in no MTRR memory type information being available for the kernel.
> 
> This has turned out to result in problems:
> 
> - Hyper-V SEV-SNP guests using uncached mappings where they shouldn't
> - Xen PV dom0 mapping memory as WB which should be UC- instead
> 
> Solve those problems by supporting to set a static MTRR state,
> overwriting the empty state used today. In case such a state has been
> set, don't call get_mtrr_state() in mtrr_bp_init(). The set state
> will only be used by mtrr_type_lookup(), as in all other cases
> mtrr_enabled() is being checked, which will return false. Accept the
> overwrite call only for selected cases when running as a guest.
> Disable X86_FEATURE_MTRR in order to avoid any MTRR modifications by
> just refusing them.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> V2:
> - new patch
> V3:
> - omit fixed MTRRs, as those are currently not needed
> - disable X86_FEATURE_MTRR instead of testing it
> - provide a stub for !CONFIG_MTRR (Michael Kelley)
> - use cpu_feature_enabled() (Boris Petkov)
> - add tests for mtrr_overwrite_state() being allowed (Boris Petkov)
> ---
>  arch/x86/include/asm/mtrr.h        |  8 ++++++
>  arch/x86/kernel/cpu/mtrr/generic.c | 43 ++++++++++++++++++++++++++++++
>  arch/x86/kernel/cpu/mtrr/mtrr.c    |  9 +++++++
>  arch/x86/kernel/setup.c            |  2 ++
>  4 files changed, 62 insertions(+)
> 
> diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
> index f0eeaf6e5f5f..f1cb81330a64 100644
> --- a/arch/x86/include/asm/mtrr.h
> +++ b/arch/x86/include/asm/mtrr.h
> @@ -31,6 +31,8 @@
>   */
>  # ifdef CONFIG_MTRR
>  void mtrr_bp_init(void);
> +void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
> +			  mtrr_type def_type);
>  extern u8 mtrr_type_lookup(u64 addr, u64 end, u8 *uniform);
>  extern void mtrr_save_fixed_ranges(void *);
>  extern void mtrr_save_state(void);
> @@ -48,6 +50,12 @@ void mtrr_disable(void);
>  void mtrr_enable(void);
>  void mtrr_generic_set_state(void);
>  #  else
> +static inline void mtrr_overwrite_state(struct mtrr_var_range *var,
> +					unsigned int num_var,
> +					mtrr_type def_type)
> +{
> +}
> +
>  static inline u8 mtrr_type_lookup(u64 addr, u64 end, u8 *uniform)
>  {
>  	/*
> diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
> index ee09d359e08f..40c59d522f57 100644
> --- a/arch/x86/kernel/cpu/mtrr/generic.c
> +++ b/arch/x86/kernel/cpu/mtrr/generic.c
> @@ -8,10 +8,12 @@
>  #include <linux/init.h>
>  #include <linux/io.h>
>  #include <linux/mm.h>
> +#include <linux/cc_platform.h>
> 
>  #include <asm/processor-flags.h>
>  #include <asm/cacheinfo.h>
>  #include <asm/cpufeature.h>
> +#include <asm/hypervisor.h>
>  #include <asm/tlbflush.h>
>  #include <asm/mtrr.h>
>  #include <asm/msr.h>
> @@ -240,6 +242,47 @@ static u8 mtrr_type_lookup_variable(u64 start, u64 end, u64
> *partial_end,
>  	return mtrr_state.def_type;
>  }
> 
> +/**
> + * 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) ||
> +		    !cpu_feature_enabled(X86_FEATURE_HYPERVISOR) ||
> +		    (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP) &&

With current upstream code, this test doesn't allow Hyper-V SNP vTOM VMs
to do the overwrite, as current upstream vTOM code doesn't participate in the
cc_platform_has() mechanism.  That's being reworked in a separate patch
set.  Can you add this test to cover the SNP vTOM case?

		    !hv_is_isolation_supported() &&

This is the same test used in __set_memory_enc_dec(), for example.  You'll
have to add #include <asm/mshyperv.h>.  There's already a stub that returns
'false' so that everything works when building with CONFIG_HYPERV=n.

Once my other patch set is accepted, I'll revise this to remove use of
hv_is_isolation_supported() outside of Hyper-V specific code, and use
the newer cc_platform_has() instead.

Michael

> +		     !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 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) {
> +		/* Software overwrite of MTRR state, only for generic case. */
> +		mtrr_calc_physbits(true);
> +		init_table();
> +		pr_info("MTRRs set to read-only\n");
> +
> +		return;
> +	}
> +
>  	phys_addr = mtrr_calc_physbits(boot_cpu_has(X86_FEATURE_MTRR));
> 
>  	if (boot_cpu_has(X86_FEATURE_MTRR)) {
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 16babff771bd..0cccfeb67c3a 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1037,6 +1037,8 @@ void __init setup_arch(char **cmdline_p)
>  	/*
>  	 * VMware detection requires dmi to be available, so this
>  	 * needs to be done after dmi_setup(), for the boot CPU.
> +	 * For some guest types (Xen PV, SEV-SNP, TDX) it is required to be
> +	 * called before cache_bp_init() for setting up MTRR state.
>  	 */
>  	init_hypervisor_platform();
> 
> --
> 2.35.3