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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: <20230221041440.GA1934@templeofstupid.com> References: <87h6vgov2p.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87h6vgov2p.ffs@tglx> On Mon, Feb 20, 2023 at 11:01:18PM +0100, Thomas Gleixner wrote: > On Mon, Feb 20 2023 at 09:17, Krister Johansen wrote: > > @@ -495,8 +496,7 @@ static int __init xen_tsc_safe_clocksource(void) > > /* 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) > > + if (ebx != XEN_CPUID_TSC_MODE_NEVER_EMULATE) > > return 0; > > > > return 1; > > What about removing more stupidity from that function? > > 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. -K