From 9ee4318c157b9802589b746cc340bae3142d984c Mon Sep 17 00:00:00 2001 From: "Kirill A. Shutemov" Date: Fri, 6 Oct 2023 17:45:49 +0300 Subject: [PATCH] x86/tdx: Mark TSC reliable MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit In x86 virtualization environments, including TDX, RDTSC instruction is handled without causing a VM exit, resulting in minimal overhead and jitters. On the other hand, other clock sources (such as HPET, ACPI timer, APIC, etc.) necessitate VM exits to implement, resulting in more fluctuating measurements compared to TSC. Thus, those clock sources are not effective for calibrating TSC. As a foundation, the host TSC is guaranteed to be invariant on any system which enumerates TDX support. TDX guests and the TDX module build on that foundation by enforcing: - Virtual TSC is monotonously incrementing for any single VCPU; - Virtual TSC values are consistent among all the TD’s VCPUs at the level supported by the CPU: + VMM is required to set the same TSC_ADJUST; + VMM must not modify from initial value of TSC_ADJUST before SEAMCALL; - The frequency is determined by TD configuration: + Virtual TSC frequency is specified by VMM on TDH.MNG.INIT; + Virtual TSC starts counting from 0 at TDH.MNG.INIT; The result is that a reliable TSC is a TDX architectural guarantee. Use the TSC as the only reliable clock source in TD guests, bypassing unstable calibration. This is similar to what the kernel already does in some VMWare and HyperV environments. [ dhansen: changelog tweaks ] Signed-off-by: Kirill A. Shutemov Signed-off-by: Dave Hansen Reviewed-by: Kuppuswamy Sathyanarayanan Reviewed-by: Erdem Aktas Reviewed-by: Isaku Yamahata Acked-by: Kai Huang Link: https://lore.kernel.org/all/20231006144549.2633-1-kirill.shutemov%40linux.intel.com --- arch/x86/coco/tdx/tdx.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c index 3e6dbd2199cf..2f27ae1e2c6b 100644 --- a/arch/x86/coco/tdx/tdx.c +++ b/arch/x86/coco/tdx/tdx.c @@ -816,6 +816,9 @@ void __init tdx_early_init(void) setup_force_cpu_cap(X86_FEATURE_TDX_GUEST); + /* TSC is the only reliable clock in TDX guest */ + setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE); + cc_vendor = CC_VENDOR_INTEL; tdx_parse_tdinfo(&cc_mask); cc_set_mask(cc_mask); -- 2.39.5