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

[Huang, Kai]

On Mon, 2023-03-06 at 17:34 +0100, Juergen Gross wrote:
> 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.
> 
> 
[...]

> diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
> index ee09d359e08f..49b4cc923312 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/mshyperv.h>

Is <asm/mshyperv.h> needed here?

>  #include <asm/tlbflush.h>
>  #include <asm/mtrr.h>
>  #include <asm/msr.h>
> @@ -240,6 +242,48 @@ 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).

+KVM list and KVM maintainers,

IIUC in the next patch, SEV-SNP guest only sets a synthetic MTRR w/o telling the
hypervisor (hyperv).  I think this works for SEV-SNP running on top of hyperv
because they have mutual understanding?

What about the SNP guest running on other hypervisors such as KVM?

Since this code covers TDX guest too, I think eventually it makes sense for TDX
guest to use this function too (to avoid #VE IIUC).  If want to do that, then I
think TDX guest should have the same mutual understanding with *ALL* hypervisor,
as I am not sure what's the point of making the TDX guest's MTRR behaviour
depending on specific hypervisor.

For now I don't see there's any use case for TDX guest to use non-WB memory type
(in fact, KVM always maps guest memory as WB if there's no non-coherent DMA to
the guest memory), so to me it seems it's OK to make a universal mutual
understanding that TDX guest will always have WB memory type for all memory.

But, I am not sure whether it's better to have a standard hypercall between
guest & hypervisor for this purpose so things can be more flexible?

> + * 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) &&
> +		     !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 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();
>