[PATCH v13 09/13] tsc: Use the GUEST_TSC_FREQ MSR for discovering TSC frequency

Nikunj A Dadhania
2024-10-21T05:58:15Z

Calibrating the TSC frequency using the kvmclock is not correct for
SecureTSC enabled guests. Use the platform provided TSC frequency via the
GUEST_TSC_FREQ MSR (C001_0134h).

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
---
arch/x86/include/asm/sev.h | 2 ++
arch/x86/coco/sev/core.c | 16 ++++++++++++++++
arch/x86/kernel/tsc.c | 5 +++++
3 files changed, 23 insertions(+)

diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 9169b18eeb78..34f7b9fc363b 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -536,6 +536,7 @@ static inline int handle_guest_request(struct snp_msg_desc *mdesc, u64 exit_code
}

void __init snp_secure_tsc_prepare(void);
+void __init securetsc_init(void);

#else /* !CONFIG_AMD_MEM_ENCRYPT */

@@ -584,6 +585,7 @@ static inline int handle_guest_request(struct snp_msg_desc *mdesc, u64 exit_code
u32 resp_sz) { return -ENODEV; }

static inline void __init snp_secure_tsc_prepare(void) { }
+static inline void __init securetsc_init(void) { }

#endif /* CONFIG_AMD_MEM_ENCRYPT */

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 4e9b1cc1f26b..154d568c59cf 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -3065,3 +3065,19 @@ void __init snp_secure_tsc_prepare(void)

pr_debug("SecureTSC enabled");
}
+
+static unsigned long securetsc_get_tsc_khz(void)
+{
+ unsigned long long tsc_freq_mhz;
+
+ setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+ rdmsrl(MSR_AMD64_GUEST_TSC_FREQ, tsc_freq_mhz);
+
+ return (unsigned long)(tsc_freq_mhz * 1000);
+}
+
+void __init securetsc_init(void)
+{
+ x86_platform.calibrate_cpu = securetsc_get_tsc_khz;
+ x86_platform.calibrate_tsc = securetsc_get_tsc_khz;
+}
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index dfe6847fd99e..c83f1091bb4f 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -30,6 +30,7 @@
#include <asm/i8259.h>
#include <asm/topology.h>
#include <asm/uv/uv.h>
+#include <asm/sev.h>

unsigned int __read_mostly cpu_khz; /* TSC clocks / usec, not used here */
EXPORT_SYMBOL(cpu_khz);
@@ -1514,6 +1515,10 @@ void __init tsc_early_init(void)
/* Don't change UV TSC multi-chassis synchronization */
if (is_early_uv_system())
return;
+
+ if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC))
+ securetsc_init();
+
if (!determine_cpu_tsc_frequencies(true))
return;
tsc_enable_sched_clock();
--
2.34.1