linux-kernel - Re: Cleaning up the KVM clock

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Date:	Tue, 23 Dec 2014 00:14:52 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...nel.org>, kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Cleaning up the KVM clock



On 23/12/2014 00:00, Andy Lutomirski wrote:
> On Mon, Dec 22, 2014 at 2:49 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>>
>> On 22/12/2014 17:03, Andy Lutomirski wrote:
>>> This is wrong.  The guest *kernel* might not see the intermediate
>>> state because the kernel (presumably it disabled migration while
>>> reading pvti), but the guest vdso can't do that and could very easily
>>> observe pvti while it's being written.
>>
>> No.  kvm_guest_time_update is called by vcpu_enter_guest, while the vCPU
>> is not running, so it's entirely atomic from the point of view of the guest.
> 
> Which vCPU?  Unless kvm_guest_time_update freezes all of the vcpus,
> then there's a race:
> 
> vCPU 0 guest: __getcpu
> vdso thread migrates to vCPU 1
> vCPU 0 exits
> host starts writing pvti for vCPU 0
> vdso thread starts reading pvti
> host finishes writing pvti for vCPU 0
> vCPU 0 resumes
> vdso migrates back to vCPU 0
> __getcpu returns 0
> 
> and we fail.

Yes, it does.  See kvm_gen_update_masterclock.

See also http://www.spinics.net/lists/kvm/msg95533.html for some
discussion about KVM_REQ_MCLOCK_INPROGRESS.

> I'm having a hard time testing, since KVM on 3.19-rc1 appears to be
> entirely unusable.  No matter what I do, I get this very early in
> guest boot:
> 
> KVM internal error. Suberror: 1
> emulation failure
> EAX=000dee58 EBX=00000000 ECX=00000000 EDX=00000cfd
> ESI=00000059 EDI=00000000 EBP=00000000 ESP=00006fc4
> EIP=000f17f4 EFL=00010012 [----A--] CPL=0 II=0 A20=1 SMM=0 HLT=0
> ES =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> CS =0008 00000000 ffffffff 00c09b00 DPL=0 CS32 [-RA]
> SS =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> DS =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> FS =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> GS =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> LDT=0000 00000000 0000ffff 00008200 DPL=0 LDT
> TR =0000 00000000 0000ffff 00008b00 DPL=0 TSS32-busy
> GDT=     000f6c58 00000037
> IDT=     000f6c96 00000000
> CR0=60000011 CR2=00000000 CR3=00000000 CR4=00000000
> DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000
> DR3=0000000000000000
> DR6=00000000ffff0ff0 DR7=0000000000000400
> EFER=0000000000000000
> Code=e8 75 fc ff ff 89 f2 a8 10 89 d8 75 0a b9 74 17 ff ff ff d1 <5b>
> 5e c3 5b 5e e9 76 ff ff ff 57 56 53 8b 35 38 65 0f 00 85 f6 0f 88 be
> 00 00 00 0f b7 f6
> 
> and it sometimes comes with a lockdep splat, too.

I can look at it tomorrow.  Does commit
2c4aa55a6af070262cca425745e8e54310e96b8d work for you?

Paolo
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