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. > >> I'll send patches for the whole mess, complete with lots of comments, >> after I test them a bit today. > > Ok, some comments can certainly help the discussion. > 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. --Andy > Paolo -- Andy Lutomirski AMA Capital Management, LLC -- 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
