| Message ID | 20221208163650.GA3225@templeofstupid.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [linux-next] x86/xen/time: prefer tsc as clocksource when it is invariant | expand |
On 12/8/22 11:36 AM, Krister Johansen wrote: > + /* > + * As Dom0 is never moved, no penalty on using TSC there. > + * > + * If the guest has invariant tsc, then set xen_clocksource rating > + * below that of the tsc so that the system prefers tsc instead. This > + * check excludes PV domains, because PV is unable to guarantee that the > + * guest's cpuid call has been intercepted by the hypervisor. > + */ > + if (xen_initial_domain()) { > xen_clocksource.rating = 275; > + } else if ((xen_hvm_domain() || xen_pvh_domain()) && > + boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && > + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) && > + !check_tsc_unstable()) { > + xen_clocksource.rating = 299; > + } What if RDTSC is intercepted? -boris
On Fri, Dec 09, 2022 at 02:32:15PM -0500, Boris Ostrovsky wrote: > > On 12/8/22 11:36 AM, Krister Johansen wrote: > > + /* > > + * As Dom0 is never moved, no penalty on using TSC there. > > + * > > + * If the guest has invariant tsc, then set xen_clocksource rating > > + * below that of the tsc so that the system prefers tsc instead. This > > + * check excludes PV domains, because PV is unable to guarantee that the > > + * guest's cpuid call has been intercepted by the hypervisor. > > + */ > > + if (xen_initial_domain()) { > > xen_clocksource.rating = 275; > > + } else if ((xen_hvm_domain() || xen_pvh_domain()) && > > + boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && > > + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) && > > + !check_tsc_unstable()) { > > + xen_clocksource.rating = 299; > > + } > > > What if RDTSC is intercepted? Right, thanks. I'll send out an updated patch here shortly that attempts to address this by examining the cpuid information to determine if the tsc is being emulated. -K
diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c index 9ef0a5cca96e..ca78581e4221 100644 --- a/arch/x86/xen/time.c +++ b/arch/x86/xen/time.c @@ -480,9 +480,22 @@ static void __init xen_time_init(void) int cpu = smp_processor_id(); struct timespec64 tp; - /* As Dom0 is never moved, no penalty on using TSC there */ - if (xen_initial_domain()) + /* + * As Dom0 is never moved, no penalty on using TSC there. + * + * If the guest has invariant tsc, then set xen_clocksource rating + * below that of the tsc so that the system prefers tsc instead. This + * check excludes PV domains, because PV is unable to guarantee that the + * guest's cpuid call has been intercepted by the hypervisor. + */ + if (xen_initial_domain()) { xen_clocksource.rating = 275; + } else if ((xen_hvm_domain() || xen_pvh_domain()) && + boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) && + !check_tsc_unstable()) { + xen_clocksource.rating = 299; + } clocksource_register_hz(&xen_clocksource, NSEC_PER_SEC);
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. The Xen documentation further clarifies that this is only reliable on HVM and PVH because PV cannot intercept a cpuid instruction. 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 guest is a HVM or PVH domain. [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 <kjlx@templeofstupid.com> Code-reviewed-by: David Reaver <me@davidreaver.com> --- arch/x86/xen/time.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-)