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

[Borislav Petkov]

On Sat, Apr 01, 2023 at 08:36:41AM +0200, 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:

Let's add the links to those problems:

> - Hyper-V SEV-SNP guests using uncached mappings where they shouldn't

I can't find Michael's original report, do you have a link?

> - Xen PV dom0 mapping memory as WB which should be UC- instead

Link: https://lore.kernel.org/all/4fe9541e-4d4c-2b2a-f8c8-2d34a7284930@xxxxxxxxxxxxxxx/

> 
> Solve those problems by supporting to set a static MTRR state,

s/by supporting to set a/allowing an MTRR static state override/

> +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))

WARN_ON_ONCE() is probably better.

> +		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 a SEV-SNP guest on a HyperV with vTOM enabled

that's a single condition.

> +	 * - when running as Xen PV guest
> +	 * - when running as TSX guest
> +	 */
> +	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))

IOW:

	if (!(hv_is_isolation_supported() && cc_platform_has(CC_ATTR_GUEST_SEV_SNP)) &&
	    !cpu_feature_enabled(X86_FEATURE_XENPV) &&
	    !cpu_feature_enabled(X86_FEATURE_TDX_GUEST))


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette