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Peter Anvin" , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Marcelo Tosatti , Anthony Liguori , David Reaver , Brendan Gregg Subject: [PATCH linux-next v3] x86/xen/time: prefer tsc as clocksource when it is invariant Message-ID: <20221216162118.GB2633@templeofstupid.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Kvm elects to use tsc instead of kvm-clock when it can detect that the TSC is invariant. (As of commit 7539b174aef4 ("x86: kvmguest: use TSC clocksource if invariant TSC is exposed")). Notable cloud vendors[1] and performance engineers[2] recommend that Xen users preferentially select tsc over xen-clocksource due the performance penalty incurred by the latter. These articles are persuasive and tailored to specific use cases. In order to understand the tradeoffs around this choice more fully, this author had to reference the documented[3] complexities around the Xen configuration, as well as the kernel's clocksource selection algorithm. Many users may not attempt this to correctly configure the right clock source in their guest. The approach taken in the kvm-clock module spares users this confusion, where possible. Both the Intel SDM[4] and the Xen tsc documentation explain that marking a tsc as invariant means that it should be considered stable by the OS and is elibile to be used as a wall clock source. In order to obtain better out-of-the-box performance, and reduce the need for user tuning, follow kvm's approach and decrease the xen clock rating so that tsc is preferable, if it is invariant, stable, and the tsc will never be emulated. [1] https://aws.amazon.com/premiumsupport/knowledge-center/manage-ec2-linux-clock-source/ [2] https://www.brendangregg.com/blog/2021-09-26/the-speed-of-time.html [3] https://xenbits.xen.org/docs/unstable/man/xen-tscmode.7.html [4] Intel 64 and IA-32 Architectures Sofware Developer's Manual Volume 3b: System Programming Guide, Part 2, Section 17.17.1, Invariant TSC Signed-off-by: Krister Johansen Code-reviewed-by: David Reaver --- v3: - Temporarily drop references to public API headers - tsc_emulated check is redundant if tsc_mode is no emulate. (feedback from Boris Ostrovsky) - Use cpuid_count instead of cpuid when accessing a cpuid leaf with subleaves. (feedback from Jan Beulich) - Kernel has its own cpuid hypercall and can intercept checks about the tsc mode. The check for PV mode is inapposite and therefore removed. (feedback from Jan Beulich) v2: - Use cpuid information to determine if tsc is emulated. Do not use tsc as clocksource if it is. (feedback from Boris Ostrovsky) - Move tsc checks into their own helper function - Add defines for tsc cpuid flags needed by new helper function. --- arch/x86/xen/time.c | 38 +++++++++++++++++++++++++++++++++++++- 1 file changed, 37 insertions(+), 1 deletion(-) diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c index 9ef0a5cca96e..95140609c8a8 100644 --- a/arch/x86/xen/time.c +++ b/arch/x86/xen/time.c @@ -474,15 +474,51 @@ static void xen_setup_vsyscall_time_info(void) xen_clocksource.vdso_clock_mode = VDSO_CLOCKMODE_PVCLOCK; } +/* + * Check if it is possible to safely use the tsc as a clocksource. This is + * only true if the hypervisor notifies the guest that its tsc is invariant, + * the tsc is stable, and the tsc instruction will never be emulated. + */ +static int __init xen_tsc_safe_clocksource(void) +{ + u32 eax, ebx, ecx, edx; + + if (!(boot_cpu_has(X86_FEATURE_CONSTANT_TSC))) + return 0; + + if (!(boot_cpu_has(X86_FEATURE_NONSTOP_TSC))) + return 0; + + if (check_tsc_unstable()) + return 0; + + /* Leaf 4, sub-leaf 0 (0x40000x03) */ + cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx); + + /* tsc_mode = no_emulate (2) */ + if (ebx != 2) + return 0; + + return 1; +} + static void __init xen_time_init(void) { struct pvclock_vcpu_time_info *pvti; int cpu = smp_processor_id(); struct timespec64 tp; - /* As Dom0 is never moved, no penalty on using TSC there */ + /* + * As Dom0 is never moved, no penalty on using TSC there. + * + * If it is possible for the guest to determine that the tsc is a safe + * clocksource, then set xen_clocksource rating below that of the tsc + * so that the system prefers tsc instead. + */ if (xen_initial_domain()) xen_clocksource.rating = 275; + else if (xen_tsc_safe_clocksource()) + xen_clocksource.rating = 299; clocksource_register_hz(&xen_clocksource, NSEC_PER_SEC); -- 2.25.1