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

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Date:   Tue, 22 Sep 2020 18:39:16 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...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 22/09/20 17:39, Maxim Levitsky wrote:
>>> I'll talk to Maxim and see if he can work on the kvmclock migration stuff.
> 
> We talked about this on IRC and now I am also convinced that we should implement
> proper TSC migration instead, so I guess I'll drop this patch and I will implement it.
> 
> Last few weeks I was digging through all the timing code, and I mostly understand it
> so it shouldn't take me much time to implement it.
> 
> There is hope that this will make nested migration fully stable since, with this patch,
> it still sometimes hangs. While on my AMD machine it takes about half a day of migration
> cycles to reproduce this, on my Intel's laptop even with this patch I can hang the nested
> guest after 10-20 cycles. The symptoms look very similar to the issue that this patch
> tried to fix.
>  
> Maybe we should keep the *comment* I added to document this funny TSC read behavior. 
> When I implement the whole thing, maybe I add a comment only version of this patch
> for that.

Sure, that's a good idea.

Paolo