linux-kernel - Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC

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Message-ID: <20201207233423.GE27492@fuller.cnet>
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.