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

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Date:   Tue, 22 Sep 2020 17:50:36 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.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 Tue, 2020-09-22 at 14:50 +0200, Paolo Bonzini wrote:
> 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.

Could you explain why though? After my patch, the KVM_GET_MSR will consistently
read the L1 TSC, just like all other MSRs as I explained. I guess for debugging,
this should work?

The fact that TSC reads with the guest offset is a nice exception made for the guests,
that insist on reading this msr without inteception and not using rdtsc.

Best regards,
	Maxim Levitsky

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