[v13,07/13] x86/sev: Mark Secure TSC as reliable clocksource

Commit Message

Nikunj A. Dadhania Oct. 21, 2024, 5:51 a.m. UTC

In SNP guest environment with Secure TSC enabled, unlike other clock
sources (such as HPET, ACPI timer, APIC, etc.), the RDTSC instruction is
handled without causing a VM exit, resulting in minimal overhead and
jitters. Hence, mark Secure TSC as the only reliable clock source,
bypassing unstable calibration.

Signed-off-by: Nikunj A Dadhania <[e-mail]>
Tested-by: Peter Gonda <[e-mail]>
Reviewed-by: Tom Lendacky <[e-mail]>
---
 arch/x86/mm/mem_encrypt_amd.c | 4 ++++
 1 file changed, 4 insertions(+)

Patch

diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
index 86a476a426c2..e9fb5f24703a 100644
--- a/arch/x86/mm/mem_encrypt_amd.c
+++ b/arch/x86/mm/mem_encrypt_amd.c
@@ -516,6 +516,10 @@  void __init sme_early_init(void)
 	 * kernel mapped.
 	 */
 	snp_update_svsm_ca();
+
+	/* Mark the TSC as reliable when Secure TSC is enabled */
+	if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
+		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
 }
 
 void __init mem_encrypt_free_decrypted_mem(void)