Should we perhaps CC qemu-devel here for an opinion. Guys, this mail should explain the issue but in case there are questions, the whole thread starts here:
http://lkml.kernel.org/r/20140407111725.GC25152@localhost Thanks. On Sat, Apr 12, 2014 at 01:35:49AM +0800, Jet Chen wrote: > On 04/12/2014 12:33 AM, H. Peter Anvin wrote: > > On 04/11/2014 06:51 AM, Romer, Benjamin M wrote: > >> > >>> I'm still confused where KVM comes into the picture. Are you actually > >>> using KVM (and thus talking about nested virtualization) or are you > >>> using Qemu in JIT mode and running another hypervisor underneath? > >> > >> The test that Fengguang used to find the problem was running the linux > >> kernel directly using KVM. When the kernel was run with "-cpu Haswell, > >> +smep,+smap" set, the vmcall failed with invalid op, but when the kernel > >> is run with "-cpu qemu64", the vmcall causes a vmexit, as it should. > > > > As far as I know, Fengguang's test doesn't use KVM at all, it runs Qemu > > as a JIT. Completely different thing. In that case Qemu probably > > should *not* set the hypervisor bit. However, the only thing that the > > hypervisor bit means is that you can look for specific hypervisor APIs > > in CPUID level 0x40000000+. > > > >> My point is, the vmcall was made because the hypervisor bit was set. If > >> this bit had been turned off, as it would be on a real processor, the > >> vmcall wouldn't have happened. > > > > And my point is that that is a bug. In the driver. A very serious one. > > You cannot call VMCALL until you know *which* hypervisor API(s) you > > have available, period. > > > >>> The hypervisor bit is a complete red herring. If the guest CPU is > >>> running in VT-x mode, then VMCALL should VMEXIT inside the guest > >>> (invoking the guest root VT-x), > >> > >> The CPU is running in VT-X. That was my point, the kernel is running in > >> the KVM guest, and KVM is setting the CPU feature bits such that bit 31 > >> is enabled. > > > > Which it is because it wants to export the KVM hypercall interface. > > However, keying VMCALL *only* on the HYPERVISOR bit is wrong in the extreme. > > > >> I don't think it's a red herring because the kernel uses this bit > >> elsewhere - it is reported as X86_FEATURE_HYPERVISOR in the CPU > >> features, and can be checked with the cpu_has_hypervisor macro (which > >> was not used by the original author of the code in the driver, but > >> should have been). VMWare and KVM support in the kernel also check for > >> this bit before checking their hypervisor leaves for an ID. If it's not > >> properly set it affects more than just the s-Par drivers. > >> > >>> but the fact still remains that you > >>> should never, ever, invoke VMCALL unless you know what hypervisor you > >>> have underneath. > >> > >> From the standpoint of the s-Par drivers, yes, I agree (as I already > >> said). However, VMCALL is not a privileged instruction, so anyone could > >> use it from user space and go right past the OS straight to the > >> hypervisor. IMHO, making it *lethal* to the guest is a bad idea, since > >> any user could hard-stop the guest with a couple of lines of C. > > > > Typically the hypervisor wants to generate a #UD inside of the guest for > > that case. The guest OS will intercept it and SIGILL the user space > > process. > > > > -hpa > > > > Hi Ben, > > I re-tested this case with/without option -enable-kvm. > > qemu-system-x86_64 -cpu Haswell,+smep,+smap invalid op > qemu-system-x86_64 -cpu kvm64 invalid op > qemu-system-x86_64 -cpu Haswell,+smep,+smap -enable-kvm everything OK > qemu-system-x86_64 -cpu kvm64 -enable-kvm everything OK > > I think this is probably a bug in QEMU. > Sorry for misleading you. I am not experienced in QEMU usage. I don't realize > I need try this case with different options Until read Peter's reply. > > As Peter said, QEMU probably should *not* set the hypervisor bit. But based > on my testing, I think KVM works properly in this case. > > Thanks, > Jet > -- Regards/Gruss, Boris. Sent from a fat crate under my desk. Formatting is fine. --
