[PATCH] x86/tdx: Override the tsc calibration for TDX VMs

[Vishal Annapurve <vannapurve@google.com>]

TSC calibration for native execution gets the TSC frequency from CPUID,
but also ends up setting lapic_timer_period. When using oneshot mode
with lapic timer, predefined value of lapic_timer_period causes lapic
timer calibration to be skipped with wrong multipliers set for lapic
timer.

To avoid this issue, override the TSC calibration step for TDX VMs to
just calculate the TSC frequency using cpuid values.

Signed-off-by: Vishal Annapurve <vannapurve@google.com>
---
 arch/x86/coco/tdx/tdx.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 1d6b863c42b0..6625594f8c62 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -757,6 +757,33 @@ static bool tdx_enc_status_change_finish(unsigned long vaddr, int numpages,
 	return true;
 }
 
+/**
+ * Determine TSC frequency via CPUID, else return 0.
+ */
+static unsigned long tdx_calibrate_tsc(void)
+{
+	unsigned int eax_denominator = 0, ebx_numerator = 0, ecx_hz = 0, edx = 0;
+	unsigned int crystal_khz;
+
+	/* CPUID 15H TSC/Crystal ratio, plus optionally Crystal Hz */
+	cpuid(0x15, &eax_denominator, &ebx_numerator, &ecx_hz, &edx);
+
+	if (ebx_numerator == 0 || eax_denominator == 0)
+		return 0;
+
+	crystal_khz = ecx_hz / 1000;
+
+	/*
+	 * TSC frequency reported directly by CPUID is a "hardware reported"
+	 * frequency and is the most accurate one so far we have. This
+	 * is considered a known frequency.
+	 */
+	if (crystal_khz != 0)
+		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+
+	return crystal_khz * ebx_numerator / eax_denominator;
+}
+
 void __init tdx_early_init(void)
 {
 	u64 cc_mask;
@@ -808,6 +835,7 @@ void __init tdx_early_init(void)
 
 	x86_platform.guest.enc_cache_flush_required  = tdx_cache_flush_required;
 	x86_platform.guest.enc_tlb_flush_required    = tdx_tlb_flush_required;
+	x86_platform.calibrate_tsc = tdx_calibrate_tsc;
 
 	/*
 	 * TDX intercepts the RDMSR to read the X2APIC ID in the parallel
-- 
2.42.0.609.gbb76f46606-goog