linux-kernel - Re: [PATCH v2 1/1] KVM: x86: fix MSR_IA32

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Message-ID: <de9411ce-aa83-77c8-b2ae-a3873250a0b1@redhat.com>
Date:   Tue, 22 Sep 2020 14:50:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 1/1] KVM: x86: fix MSR_IA32_TSC read for nested
 migration

On 21/09/20 18:23, Sean Christopherson wrote:
> Avoid "should" in code comments and describe what the code is doing, not what
> it should be doing.  The only exception for this is when the code has a known
> flaw/gap, e.g. "KVM should do X, but because of Y, KVM actually does Z".
> 
>> +		 * return it's real L1 value so that its restore will be correct.
> s/it's/its
> 
> Perhaps add "unconditionally" somewhere, since arch.tsc_offset can also contain
> the L1 value.  E.g. 
> 
> 		 * Unconditionally return L1's TSC offset on userspace reads
> 		 * so that userspace reads and writes always operate on L1's
> 		 * offset, e.g. to ensure deterministic behavior for migration.
> 		 */
> 

Technically the host need not restore MSR_IA32_TSC at all.  This follows
the idea of the discussion with Oliver Upton about transmitting the
state of the kvmclock heuristics to userspace, which include a (TSC,
CLOCK_MONOTONIC) pair to transmit the offset to the destination.  All
that needs to be an L1 value is then the TSC value in that pair.

I'm a bit torn over this patch.  On one hand it's an easy solution, on
the other hand it's... just wrong if KVM_GET_MSR is used for e.g.
debugging the guest.

I'll talk to Maxim and see if he can work on the kvmclock migration stuff.

Paolo