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SubjectRe: [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@intel.com>

Reviewed-by: Chao Gao <chao.gao@intel.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?

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