[PATCH v4 07/47] x86/tdx: Force TSC frequency with CPUID-based info provided by the TDX-Module

[Sean Christopherson <seanjc@xxxxxxxxxx>]

When running as a TDX guest, explicitly set the TSC frequency to a known
value, using CPUID-based information, instead of potentially relying on a
hypervisor-controlled PV routine.  For TDX guests, CPUID.0x15 is always
emulated by the TDX-Module, i.e. the information from CPUID is more
trustworthy than the information provided by the hypervisor.

To maintain backwards compatibility with TDX guest kernels that use native
calibration, and because it's the least awful option, retain
native_calibrate_tsc()'s stuffing of the local APIC bus period using the
core crystal frequency.  While it's entirely possible for the hypervisor
to emulate the APIC timer at a different frequency than the core crystal
frequency, the commonly accepted interpretation of Intel's SDM is that APIC
timer runs at the core crystal frequency when that latter is enumerated via
CPUID:

  The APIC timer frequency will be the processor’s bus clock or core
  crystal clock frequency (when TSC/core crystal clock ratio is enumerated
  in CPUID leaf 0x15).

If the hypervisor is malicious and deliberately runs the APIC timer at the
wrong frequency, nothing would stop the hypervisor from modifying the
frequency at any time, i.e. attempting to manually calibrate the frequency
out of paranoia would be futile.

Deliberately leave CPU frequency calibration as is, since the TDX-Module
doesn't provide any guarantees with respect to CPUID.0x16.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/coco/tdx/tdx.c    | 20 +++++++++++++++++---
 arch/x86/include/asm/tdx.h |  2 ++
 arch/x86/kernel/tsc.c      |  3 +++
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 29b6f1ed59ec..5d7976359220 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -8,6 +8,7 @@
 #include <linux/export.h>
 #include <linux/io.h>
 #include <linux/kexec.h>
+#include <asm/apic.h>
 #include <asm/coco.h>
 #include <asm/tdx.h>
 #include <asm/vmx.h>
@@ -1123,9 +1124,6 @@ void __init tdx_early_init(void)
 
        setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
 
-       /* TSC is the only reliable clock in TDX guest */
-       setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
-
        cc_vendor = CC_VENDOR_INTEL;
 
        /* Configure the TD */
@@ -1195,3 +1193,19 @@ void __init tdx_early_init(void)
 
        tdx_announce();
 }
+
+unsigned int __init tdx_tsc_init(void)
+{
+       struct cpuid_tsc_info info;
+
+       if (WARN_ON_ONCE(cpuid_get_tsc_freq(&info)))
+               return 0;
+
+       lapic_timer_period = info.crystal_khz * 1000 / HZ;
+
+       /* TSC is the only reliable clock in TDX guest */
+       setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
+       setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+
+       return info.tsc_khz;
+}
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index e5a9cf656c07..1d841d464aa4 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -67,6 +67,7 @@ struct ve_info {
 #ifdef CONFIG_INTEL_TDX_GUEST
 
 void __init tdx_early_init(void);
+unsigned int __init tdx_tsc_init(void);
 
 void tdx_get_ve_info(struct ve_info *ve);
 
@@ -88,6 +89,7 @@ void __init tdx_dump_td_ctls(u64 td_ctls);
 #else
 
 static inline void tdx_early_init(void) { };
+static inline unsigned int tdx_tsc_init(void) { return 0; }
 static inline void tdx_halt(void) { };
 
 static inline bool tdx_early_handle_ve(struct pt_regs *regs) { return false; }
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 2b8f94c3fcc7..2603f136e29b 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -34,6 +34,7 @@
 #include <asm/topology.h>
 #include <asm/uv/uv.h>
 #include <asm/sev.h>
+#include <asm/tdx.h>
 
 unsigned int __read_mostly cpu_khz;    /* TSC clocks / usec, not used here */
 EXPORT_SYMBOL(cpu_khz);
@@ -1550,6 +1551,8 @@ void __init tsc_early_init(void)
                known_tsc_khz = tsc_early_khz;
        else if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC))
                known_tsc_khz = snp_secure_tsc_init();
+       else if (boot_cpu_has(X86_FEATURE_TDX_GUEST))
+               known_tsc_khz = tdx_tsc_init();
 
        if (!determine_cpu_tsc_frequencies(true, known_tsc_khz))
                return;
-- 
2.54.0.823.g6e5bcc1fc9-goog