linux-kernel - Re: [PATCH v3 01/21] x86/virt/tdx: Detect SEAM

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Date:   Tue, 26 Apr 2022 13:21:30 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v3 01/21] x86/virt/tdx: Detect SEAM

> +config INTEL_TDX_HOST
> +	bool "Intel Trust Domain Extensions (TDX) host support"
> +	default n
> +	depends on CPU_SUP_INTEL
> +	depends on X86_64
> +	help
> +	  Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
> +	  host and certain physical attacks.  This option enables necessary TDX
> +	  support in host kernel to run protected VMs.
> +
> +	  If unsure, say N.

Nothing about KVM?

...
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> new file mode 100644
> index 000000000000..03f35c75f439
> --- /dev/null
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -0,0 +1,102 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright(c) 2022 Intel Corporation.
> + *
> + * Intel Trusted Domain Extensions (TDX) support
> + */
> +
> +#define pr_fmt(fmt)	"tdx: " fmt
> +
> +#include <linux/types.h>
> +#include <linux/cpumask.h>
> +#include <asm/msr-index.h>
> +#include <asm/msr.h>
> +#include <asm/cpufeature.h>
> +#include <asm/cpufeatures.h>
> +#include <asm/tdx.h>
> +
> +/* Support Intel Secure Arbitration Mode Range Registers (SEAMRR) */
> +#define MTRR_CAP_SEAMRR			BIT(15)
> +
> +/* Core-scope Intel SEAMRR base and mask registers. */
> +#define MSR_IA32_SEAMRR_PHYS_BASE	0x00001400
> +#define MSR_IA32_SEAMRR_PHYS_MASK	0x00001401
> +
> +#define SEAMRR_PHYS_BASE_CONFIGURED	BIT_ULL(3)
> +#define SEAMRR_PHYS_MASK_ENABLED	BIT_ULL(11)
> +#define SEAMRR_PHYS_MASK_LOCKED		BIT_ULL(10)
> +
> +#define SEAMRR_ENABLED_BITS	\
> +	(SEAMRR_PHYS_MASK_ENABLED | SEAMRR_PHYS_MASK_LOCKED)
> +
> +/* BIOS must configure SEAMRR registers for all cores consistently */
> +static u64 seamrr_base, seamrr_mask;
> +
> +static bool __seamrr_enabled(void)
> +{
> +	return (seamrr_mask & SEAMRR_ENABLED_BITS) == SEAMRR_ENABLED_BITS;
> +}

But there's no case where seamrr_mask is non-zero and where
_seamrr_enabled().  Why bother checking the SEAMRR_ENABLED_BITS?

> +static void detect_seam_bsp(struct cpuinfo_x86 *c)
> +{
> +	u64 mtrrcap, base, mask;
> +
> +	/* SEAMRR is reported via MTRRcap */
> +	if (!boot_cpu_has(X86_FEATURE_MTRR))
> +		return;
> +
> +	rdmsrl(MSR_MTRRcap, mtrrcap);
> +	if (!(mtrrcap & MTRR_CAP_SEAMRR))
> +		return;
> +
> +	rdmsrl(MSR_IA32_SEAMRR_PHYS_BASE, base);
> +	if (!(base & SEAMRR_PHYS_BASE_CONFIGURED)) {
> +		pr_info("SEAMRR base is not configured by BIOS\n");
> +		return;
> +	}
> +
> +	rdmsrl(MSR_IA32_SEAMRR_PHYS_MASK, mask);
> +	if ((mask & SEAMRR_ENABLED_BITS) != SEAMRR_ENABLED_BITS) {
> +		pr_info("SEAMRR is not enabled by BIOS\n");
> +		return;
> +	}
> +
> +	seamrr_base = base;
> +	seamrr_mask = mask;
> +}

Comment, please.

	/*
	 * Stash the boot CPU's MSR values so that AP values
	 * can can be checked for consistency.
	 */


> +static void detect_seam_ap(struct cpuinfo_x86 *c)
> +{
> +	u64 base, mask;
> +
> +	/*
> +	 * Don't bother to detect this AP if SEAMRR is not
> +	 * enabled after earlier detections.
> +	 */
> +	if (!__seamrr_enabled())
> +		return;
> +
> +	rdmsrl(MSR_IA32_SEAMRR_PHYS_BASE, base);
> +	rdmsrl(MSR_IA32_SEAMRR_PHYS_MASK, mask);
> +

This is the place for a comment about why the values have to be equal.

> +	if (base == seamrr_base && mask == seamrr_mask)
> +		return;
> +
> +	pr_err("Inconsistent SEAMRR configuration by BIOS\n");
> +	/* Mark SEAMRR as disabled. */
> +	seamrr_base = 0;
> +	seamrr_mask = 0;
> +}
> +
> +static void detect_seam(struct cpuinfo_x86 *c)
> +{
> +	if (c == &boot_cpu_data)
> +		detect_seam_bsp(c);
> +	else
> +		detect_seam_ap(c);
> +}
> +
> +void tdx_detect_cpu(struct cpuinfo_x86 *c)
> +{
> +	detect_seam(c);
> +}

The extra function looks a bit silly here now.  Maybe this gets filled
out later, but it's goofy-looking here.