linux-kernel - Re: [PATCH v5 01/22] x86/virt/tdx: Detect TDX during kernel boot

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Date:   Thu, 23 Jun 2022 13:57:03 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        seanjc@...gle.com, pbonzini@...hat.com, dave.hansen@...el.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 v5 01/22] x86/virt/tdx: Detect TDX during kernel boot

On Wed, Jun 22, 2022 at 11:15:30PM +1200, Kai Huang wrote:
>Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
>host and certain physical attacks.  TDX introduces a new CPU mode called
>Secure Arbitration Mode (SEAM) and a new isolated range pointed by the
						    ^ perhaps, range of memory

>SEAM Ranger Register (SEAMRR).  A CPU-attested software module called
>'the TDX module' runs inside the new isolated range to implement the
>functionalities to manage and run protected VMs.
>
>Pre-TDX Intel hardware has support for a memory encryption architecture
>called MKTME.  The memory encryption hardware underpinning MKTME is also
>used for Intel TDX.  TDX ends up "stealing" some of the physical address
>space from the MKTME architecture for crypto-protection to VMs.  BIOS is
>responsible for partitioning the "KeyID" space between legacy MKTME and
>TDX.  The KeyIDs reserved for TDX are called 'TDX private KeyIDs' or
>'TDX KeyIDs' for short.
>
>To enable TDX, BIOS needs to configure SEAMRR (core-scope) and TDX
>private KeyIDs (package-scope) consistently for all packages.  TDX
>doesn't trust BIOS.  TDX ensures all BIOS configurations are correct,
>and if not, refuses to enable SEAMRR on any core.  This means detecting
>SEAMRR alone on BSP is enough to check whether TDX has been enabled by
>BIOS.
>
>To start to support TDX, create a new arch/x86/virt/vmx/tdx/tdx.c for
>TDX host kernel support.  Add a new Kconfig option CONFIG_INTEL_TDX_HOST
>to opt-in TDX host kernel support (to distinguish with TDX guest kernel
>support).  So far only KVM is the only user of TDX.  Make the new config
>option depend on KVM_INTEL.
>
>Use early_initcall() to detect whether TDX is enabled by BIOS during
>kernel boot, and add a function to report that.  Use a function instead
>of a new CPU feature bit.  This is because the TDX module needs to be
>initialized before it can be used to run any TDX guests, and the TDX
>module is initialized at runtime by the caller who wants to use TDX.
>
>Explicitly detect SEAMRR but not just only detect TDX private KeyIDs.
>Theoretically, a misconfiguration of TDX private KeyIDs can result in
>SEAMRR being disabled, but the BSP can still report the correct TDX
>KeyIDs.  Such BIOS bug can be caught when initializing the TDX module,
>but it's better to do more detection during boot to provide a more
>accurate result.
>
>Also detect the TDX KeyIDs.  This allows userspace to know how many TDX
>guests the platform can run w/o needing to wait until TDX is fully
>functional.
>
>Signed-off-by: Kai Huang <kai.huang@...el.com>

Reviewed-by: Chao Gao <chao.gao@...el.com>

But some cosmetic comments below ...

>---
>+
>+static u32 tdx_keyid_start __ro_after_init;
>+static u32 tdx_keyid_num __ro_after_init;
>+
...

>+static int detect_tdx_keyids(void)
>+{
>+	u64 keyid_part;
>+
>+	rdmsrl(MSR_IA32_MKTME_KEYID_PARTITIONING, keyid_part);

how about:
	rdmsr(MSR_IA32_MKTME_KEYID_PARTITIONING, tdx_keyid_start, tdx_keyid_num);
	tdx_keyid_start++;

Then TDX_KEYID_NUM/START can be dropped.

>+
>+	tdx_keyid_num = TDX_KEYID_NUM(keyid_part);
>+	tdx_keyid_start = TDX_KEYID_START(keyid_part);
>+
>+	pr_info("TDX private KeyID range: [%u, %u).\n",
>+			tdx_keyid_start, tdx_keyid_start + tdx_keyid_num);
>+
>+	/*
>+	 * TDX guarantees at least two TDX KeyIDs are configured by
>+	 * BIOS, otherwise SEAMRR is disabled.  Invalid TDX private
>+	 * range means kernel bug (TDX is broken).

Maybe it is better to have a comment for why TDX/kernel guarantees
there should be at least 2 TDX keyIDs.

>+
>+/*
>+ * This file contains both macros and data structures defined by the TDX
>+ * architecture and Linux defined software data structures and functions.
>+ * The two should not be mixed together for better readability.  The
>+ * architectural definitions come first.
>+ */
>+
>+/*
>+ * Intel Trusted Domain CPU Architecture Extension spec:
>+ *
>+ * IA32_MTRRCAP:
>+ *   Bit 15:	The support of SEAMRR
>+ *
>+ * IA32_SEAMRR_PHYS_MASK (core-scope):
>+ *   Bit 10:	Lock bit
>+ *   Bit 11:	Enable bit
>+ */
>+#define MTRR_CAP_SEAMRR			BIT_ULL(15)

Can you move this bit definition to arch/x86/include/asm/msr-index.h
right after MSR_MTRRcap definition there?