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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/