On 23/12/2014 00:00, Andy Lutomirski wrote: > On Mon, Dec 22, 2014 at 2:49 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
