[PATCH v15 09/13] tsc: Use the GUEST_TSC_FREQ MSR for discovering TSC frequency

Nikunj A Dadhania
2024-12-03T09:01:44Z

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 9fd02efef08e..c4dca06b3b01 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -493,6 +493,7 @@ int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req
struct snp_guest_request_ioctl *rio);

void __init snp_secure_tsc_prepare(void);
+void __init snp_secure_tsc_init(void);

#else /* !CONFIG_AMD_MEM_ENCRYPT */

@@ -536,6 +537,7 @@ static inline void snp_msg_free(struct snp_msg_desc *mdesc) { }
static inline int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req,
struct snp_guest_request_ioctl *rio) { return -ENODEV; }
static inline void __init snp_secure_tsc_prepare(void) { }
+static inline void __init snp_secure_tsc_init(void) { }

#endif /* CONFIG_AMD_MEM_ENCRYPT */

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 59c5e716fdd1..1bc668883058 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -3279,3 +3279,19 @@ 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 snp_secure_tsc_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 67aeaba4ba9c..c0eef924b84e 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);
@@ -1515,6 +1516,10 @@ 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))
+ snp_secure_tsc_init();
+
if (!determine_cpu_tsc_frequencies(true))
return;
tsc_enable_sched_clock();
--
2.34.1