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Peter Anvin" , Marcelo Tosatti , Anthony Liguori , David Reaver , Brendan Gregg Subject: Re: [PATCH linux-next 2/2] x86/xen/time: cleanup xen_tsc_safe_clocksource Message-ID: <20230221172153.GA1958@templeofstupid.com> References: <87h6vgov2p.ffs@tglx> <20230221041440.GA1934@templeofstupid.com> <20230221055117.GA1934@templeofstupid.com> <871qmjpegh.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <871qmjpegh.ffs@tglx> On Tue, Feb 21, 2023 at 10:14:54AM +0100, Thomas Gleixner wrote: > On Mon, Feb 20 2023 at 21:51, Krister Johansen wrote: > > On Mon, Feb 20, 2023 at 08:14:40PM -0800, Krister Johansen wrote: > >> > static bool __init xen_tsc_safe_clocksource(void) > >> > { > >> > u32 eax, ebx. ecx, edx; > >> > > >> > /* Leaf 4, sub-leaf 0 (0x40000x03) */ > >> > cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx); > >> > > >> > return ebx == XEN_CPUID_TSC_MODE_NEVER_EMULATE; > >> > } > >> > >> I'm all for simplifying. I'm happy to clean up that return to be more > >> idiomatic. I was under the impression, perhaps mistaken, though, that > >> the X86_FEATURE_CONSTANT_TSC, X86_FEATURE_NONSTOP_TSC, and > >> check_tsc_unstable() checks were actually serving a purpose: to ensure > >> that we don't rely on the tsc in environments where it's being emulated > >> and the OS would be better served by using a PV clock. Specifically, > >> kvmclock_init() makes a very similar set of checks that I also thought > >> were load-bearing. > > > > Bah, what I meant to say was emulated, unstable, or otherwise unsuitable > > for use as a clocksource. IOW, even if TSC_MODE_NEVER_EMULATE is > > set, it's possible that a user is attempting a migration from a cpu > > that's not invariant, and we'd still want to check for that case and > > fall back to a PV clocksource, correct? > > Sure. But a life migration from a NEVER_EMULATE to a non-invariant host > is a patently bad idea and has nothing to do with the __init function, > which is gone at that point already. > > What I wanted to say: > > static bool __init xen_tsc_safe_clocksource(void) > { > ...... > > /* Leaf 4, sub-leaf 0 (0x40000x03) */ > cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx); > > return ebx == XEN_CPUID_TSC_MODE_NEVER_EMULATE; > } Thanks, I'm happy to make it look like ^ that. I should have thought to do this myself. :/ I'll send out a v2 making this correction. > I didn't have the full context and was just looking at the condition. > Now I checked the full context and I think that except for the > > if (check_tsc_unstable()) > > check everything else can go away unless you do not trust the hypervisor > that it only sets the NEVER_EMULATE bit when CONSTANT and NONSTOP are > set as well. But yeah, you might prefer to be paranoid. It's virt after > all. Unless there are objections, I think I'd prefer to err on the side of paranoia here. Sorry for the confusion. -K