[PATCH v4 06/47] x86/sev: Shove SNP's secure/trusted TSC frequency directly into "calibration"

[Sean Christopherson <seanjc@xxxxxxxxxx>]

As a first step towards dropping .calibrate_{cpu,tsc}() and explicitly
defining precedence/priority for "calibration" routines, pass the secure
TSC frequency obtained from SNP firmware directly to
determine_cpu_tsc_frequencies() instead of overriding the .calibrate_tsc()
hook.

Unlike the native calibration routines, all of the paravirtual overrides,
including SNP and TDX, are constant in the sense that the frequency
provided by the hypervisor or trusted firmware is fixed, known, and always
available during early boot.  More importantly, for CoCo (SNP and TDX) VMs,
it's imperative that the kernel uses the frequency provided by the trusted
firmware, not by the untrusted hypervisor.  Enforcing the priority between
sources by carefully ordering seemingly unrelated init calls, so that the
trusted override "wins", is brittle and all but impossible to follow.

While it's rather weird, deliberately prioritize tsc_early_khz over all
else to maintain existing behavior.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/coco/sev/core.c   | 14 ++++----------
 arch/x86/include/asm/sev.h |  4 ++--
 arch/x86/kernel/tsc.c      | 19 ++++++++++++-------
 3 files changed, 18 insertions(+), 19 deletions(-)

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 403dcea86452..bc5ae9ef74da 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -99,7 +99,6 @@ static const char * const sev_status_feat_names[] = {
  */
 static u64 snp_tsc_scale __ro_after_init;
 static u64 snp_tsc_offset __ro_after_init;
-static unsigned long snp_tsc_freq_khz __ro_after_init;
 
 DEFINE_PER_CPU(struct sev_es_runtime_data*, runtime_data);
 DEFINE_PER_CPU(struct sev_es_save_area *, sev_vmsa);
@@ -2014,15 +2013,10 @@ void __init snp_secure_tsc_prepare(void)
        pr_debug("SecureTSC enabled");
 }
 
-static unsigned long securetsc_get_tsc_khz(void)
-{
-       return snp_tsc_freq_khz;
-}
-
-void __init snp_secure_tsc_init(void)
+unsigned int __init snp_secure_tsc_init(void)
 {
+       unsigned long snp_tsc_freq_khz, tsc_freq_mhz;
        struct snp_secrets_page *secrets;
-       unsigned long tsc_freq_mhz;
        void *mem;
 
        mem = early_memremap_encrypted(sev_secrets_pa, PAGE_SIZE);
@@ -2043,7 +2037,7 @@ void __init snp_secure_tsc_init(void)
 
        snp_tsc_freq_khz = SNP_SCALE_TSC_FREQ(tsc_freq_mhz * 1000, 
secrets->tsc_factor);
 
-       x86_platform.calibrate_tsc = securetsc_get_tsc_khz;
-
        early_memunmap(mem, PAGE_SIZE);
+
+       return snp_tsc_freq_khz;
 }
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 594cfa19cbd4..05ebf0b73ef4 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -530,7 +530,7 @@ int snp_send_guest_request(struct snp_msg_desc *mdesc, 
struct snp_guest_req *req
 int snp_svsm_vtpm_send_command(u8 *buffer);
 
 void __init snp_secure_tsc_prepare(void);
-void __init snp_secure_tsc_init(void);
+unsigned int snp_secure_tsc_init(void);
 enum es_result savic_register_gpa(u64 gpa);
 enum es_result savic_unregister_gpa(u64 *gpa);
 u64 savic_ghcb_msr_read(u32 reg);
@@ -637,7 +637,7 @@ static inline int snp_send_guest_request(struct 
snp_msg_desc *mdesc,
                                         struct snp_guest_req *req) { return 
-ENODEV; }
 static inline int snp_svsm_vtpm_send_command(u8 *buffer) { return -ENODEV; }
 static inline void __init snp_secure_tsc_prepare(void) { }
-static inline void __init snp_secure_tsc_init(void) { }
+static inline unsigned int __init snp_secure_tsc_init(void) { return 0; }
 static inline void sev_evict_cache(void *va, int npages) {}
 static inline enum es_result savic_register_gpa(u64 gpa) { return 
ES_UNSUPPORTED; }
 static inline enum es_result savic_unregister_gpa(u64 *gpa) { return 
ES_UNSUPPORTED; }
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 833eed5c048a..2b8f94c3fcc7 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1474,15 +1474,16 @@ static int __init init_tsc_clocksource(void)
  */
 device_initcall(init_tsc_clocksource);
 
-static bool __init determine_cpu_tsc_frequencies(bool early)
+static bool __init determine_cpu_tsc_frequencies(bool early,
+                                                unsigned int known_tsc_khz)
 {
        /* Make sure that cpu and tsc are not already calibrated */
        WARN_ON(cpu_khz || tsc_khz);
 
        if (early) {
                cpu_khz = x86_platform.calibrate_cpu();
-               if (tsc_early_khz)
-                       tsc_khz = tsc_early_khz;
+               if (known_tsc_khz)
+                       tsc_khz = known_tsc_khz;
                else
                        tsc_khz = x86_platform.calibrate_tsc();
        } else {
@@ -1537,16 +1538,20 @@ static void __init tsc_enable_sched_clock(void)
 
 void __init tsc_early_init(void)
 {
+       unsigned int known_tsc_khz = 0;
+
        if (!boot_cpu_has(X86_FEATURE_TSC))
                return;
        /* Don't change UV TSC multi-chassis synchronization */
        if (is_early_uv_system())
                return;
 
-       if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC))
-               snp_secure_tsc_init();
+       if (tsc_early_khz)
+               known_tsc_khz = tsc_early_khz;
+       else if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC))
+               known_tsc_khz = snp_secure_tsc_init();
 
-       if (!determine_cpu_tsc_frequencies(true))
+       if (!determine_cpu_tsc_frequencies(true, known_tsc_khz))
                return;
        tsc_enable_sched_clock();
 }
@@ -1567,7 +1572,7 @@ void __init tsc_init(void)
 
        if (!tsc_khz) {
                /* We failed to determine frequencies earlier, try again */
-               if (!determine_cpu_tsc_frequencies(false)) {
+               if (!determine_cpu_tsc_frequencies(false, 0)) {
                        mark_tsc_unstable("could not calculate TSC khz");
                        setup_clear_cpu_cap(X86_FEATURE_TSC_DEADLINE_TIMER);
                        return;
-- 
2.54.0.823.g6e5bcc1fc9-goog