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Date: Mon, 7 Dec 2020 20:34:23 -0300 From: Marcelo Tosatti <mtosatti@...hat.com> To: Thomas Gleixner <tglx@...utronix.de> Cc: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>, Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, Jim Mattson <jmattson@...gle.com>, Wanpeng Li <wanpengli@...cent.com>, "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, Vitaly Kuznetsov <vkuznets@...hat.com>, Sean Christopherson <sean.j.christopherson@...el.com>, open list <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>, Shuah Khan <shuah@...nel.org>, Andrew Jones <drjones@...hat.com>, Oliver Upton <oupton@...gle.com>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org> Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE On Sun, Dec 06, 2020 at 05:19:16PM +0100, Thomas Gleixner wrote: > On Thu, Dec 03 2020 at 19:11, Maxim Levitsky wrote: > > + case KVM_SET_TSC_STATE: { > > + struct kvm_tsc_state __user *user_tsc_state = argp; > > + struct kvm_tsc_state tsc_state; > > + u64 host_tsc, wall_nsec; > > + > > + u64 new_guest_tsc, new_guest_tsc_offset; > > + > > + r = -EFAULT; > > + if (copy_from_user(&tsc_state, user_tsc_state, sizeof(tsc_state))) > > + goto out; > > + > > + kvm_get_walltime(&wall_nsec, &host_tsc); > > + new_guest_tsc = tsc_state.tsc; > > + > > + if (tsc_state.flags & KVM_TSC_STATE_TIMESTAMP_VALID) { > > + s64 diff = wall_nsec - tsc_state.nsec; > > + if (diff >= 0) > > + new_guest_tsc += nsec_to_cycles(vcpu, diff); > > + else > > + new_guest_tsc -= nsec_to_cycles(vcpu, -diff); > > + } > > + > > + new_guest_tsc_offset = new_guest_tsc - kvm_scale_tsc(vcpu, host_tsc); > > + kvm_vcpu_write_tsc_offset(vcpu, new_guest_tsc_offset); > > >From a timekeeping POV and the guests expectation of TSC this is > fundamentally wrong: > > tscguest = scaled(hosttsc) + offset > > The TSC has to be viewed systemwide and not per CPU. It's systemwide > used for timekeeping and for that to work it has to be synchronized. > > Why would this be different on virt? Just because it's virt or what? > > Migration is a guest wide thing and you're not migrating single vCPUs. > > This hackery just papers over he underlying design fail that KVM looks > at the TSC per vCPU which is the root cause and that needs to be fixed. It already does it: The unified TSC offset is kept at kvm->arch.cur_tsc_offset.
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