From: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx> Sent: Monday, November 25, 2024 2:25 PM > > On Fri, Nov 22, 2024 at 06:33:12PM +0000, Michael Kelley wrote: > > From: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx> Sent: Tuesday, > November 19, 2024 4:51 PM > > > > > > On Tue, Nov 12, 2024 at 07:48:06PM +0000, Michael Kelley wrote: > > > > From: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx> Sent: Tuesday, > > > November 12, 2024 10:18 AM > > > > > > > > > > Enable X86_FEATURE_TSC_RELIABLE by default as X86_FEATURE_TSC_RELIABLE > is > > > > > independent from invariant TSC and should have never been gated by the > > > > > HV_ACCESS_TSC_INVARIANT privilege. > > > > > > > > I think originally X86_FEATURE_TSC_RELIABLE was gated by the Hyper-V > > > > TSC Invariant feature because otherwise VM live migration may cause > > > > the TSC value reported by the RDTSC/RDTSCP instruction in the guest > > > > to abruptly change frequency and value. In such cases, the TSC isn't > > > > useable by the kernel or user space. > > > > > > > > Enabling the Hyper-V TSC Invariant feature fixes that by using the > > > > hardware scaling available in more recent processors to automatically > > > > fixup the TSC value returned by RDTSC/RDTSCP in the guest. > > > > > > > > Is there a practical problem that is fixed by always enabling > > > > X86_FEATURE_TSC_RELIABLE? > > > > > > > > > > The particular problem is that HV_ACCESS_TSC_INVARIANT is not set for the > > > nested root, which in turn leads to keeping tsc clocksource watchdog > > > thread and TSC sycn check timer around. > > > > I have trouble keeping all the different TSC "features" conceptually > > separate. :-( The TSC frequency not changing (and the value not > > abruptly jumping?) should already be represented by > > X86_FEATURE_TSC_CONSTANT. In the kernel, X86_FEATURE_TSC_RELIABLE > > effectively only controls whether the TSC clocksource watchdog is > > enabled, and in spite of the live migration foibles, I don't see a need > > for that watchdog in a Hyper-V VM. So maybe it's OK to always set > > X86_FEATURE_TSC_RELIABLE in a Hyper-V VM, as you have > > proposed. > > > > The "tsc_reliable" flag is also exposed to user space as part of the > > /proc/cpuinfo "flags" output, so theoretically some user space > > program could change behavior based on that flag. But that seems > > a bit far-fetched. I know there are user space programs that check > > the CPUID INVARIANT_TSC flag to know whether they can use > > the raw RDTSC instruction output to do start/stop timing. The > > Hyper-V TSC Invariant feature makes that work correctly, even > > across live migrations. > > > > It sounds to me that if X86_FEATURE_TSC_CONSTANT is available > on Hyper-V, then we can set X86_FEATURE_TSC_RELIABLE. > Is it what you are saying? > No. Sorry I wasn't clear. X86_FEATURE_TSC_CONSTANT will be set only when the Hyper-V TSC Invariant feature is enabled, so tying X86_FEATURE_TSC_RELIABLE to that is what happens now. What I'm suggesting is to take your patch "as is". In other words, always enable X86_FEATURE_TSC_RELIABLE. From what I can tell, TSC_RELIABLE is only used to disable the TSC watchdog. Since I can't see a use for the TSC watchdog in a VM, always setting TSC_RELIABLE probably makes sense. TSC_RELIABLE doesn't say anything about whether the TSC frequency might change, such as across a VM live migration. TSC_CONSTANT is what tells you that the frequency won't change. My caveat is that I don't know the history of TSC_RELIABLE. I don't see any documentation on the details of what it is supposed to convey, especially in a VM. Maybe someone on the "To:" list who knows for sure can confirm what I'm thinking. Michael
