Re: [Qemu-devel] About live migration rollback

[Dr. David Alan Gilbert]

* Gonglei (Arei) (address@hidden) wrote:
> Hi Dave,
> 
> We discussed some live migration fallback scenarios in this year's KVM forum, 
> and now I can provide another scenario, perhaps the upstream should consider 
> rolling
> back for this situation.
> 
> Environments information:
> 
> host A: cpu E5620(model WestmereEP without flag xsave)
> host B: cpu E5-2643(model SandyBridgeEP with flag xsave)
> 
> The reproduce steps is :
> 1. Start a windows 2008 vm with -cpu host(which means host-passthrough).

Well we don't guarantee migration across -cpu host - does this problem
go away if both qemu's are started with matching CPU flags
(corresponding to the Westmere) ?

> 2. Migrate the vm to host B when cr4.OSXSAVE=0.
> 3. Vm runs on host B for a while so that cr4.OSXSAVE changes to 1.
> 4. Then migrate the vm to host A successfully, but vm was paused, and qemu 
> printed log as followed:
> 
> KVM: entry failed, hardware error 0x80000021
> 
> If you're running a guest on an Intel machine without unrestricted mode
> support, the failure can be most likely due to the guest entering an invalid
> state for Intel VT. For example, the guest maybe running in big real mode
> which is not supported on less recent Intel processors.
> 
> EAX=019b3bb0 EBX=01a3ae80 ECX=01a61ce8 EDX=00000000
> ESI=01a62000 EDI=00000000 EBP=00000000 ESP=01718b20
> EIP=0185d982 EFL=00000286 [--S--P-] CPL=0 II=0 A20=1 SMM=0 HLT=0
> ES =0000 00000000 0000ffff 00009300
> CS =f000 ffff0000 0000ffff 00009b00
> SS =0000 00000000 0000ffff 00009300
> DS =0000 00000000 0000ffff 00009300
> FS =0000 00000000 0000ffff 00009300
> GS =0000 00000000 0000ffff 00009300
> LDT=0000 00000000 0000ffff 00008200
> TR =0000 00000000 0000ffff 00008b00
> GDT=     00000000 0000ffff
> IDT=     00000000 0000ffff
> CR0=60000010 CR2=00000000 CR3=00000000 CR4=00000000
> DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 
> DR3=0000000000000000
> DR6=00000000ffff0ff0 DR7=0000000000000400
> EFER=0000000000000000
> Code=00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <00> 00 00 
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
> 00
> 
> Problem happened when kvm_put_sregs returns err -22(called by 
> kvm_arch_put_registers(qemu)).
> 
> Because kvm_arch_vcpu_ioctl_set_sregs(kvm module) checked that 
> guest_cpuid_has no X86_FEATURE_XSAVE but cr4.OSXSAVE=1.
> We should cancel migration if kvm_arch_put_registers returns error.

Do you have a backtrace of when the kvm_arch_put_registers is called
when it fails?
If it's called during the loading of the device state then we should be
able to detect it and fail the migration; however if it's only failing
after the CPU is restarted after the migration then it's a bit too late.

Dave

> Thanks,
> -Gonglei
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK