[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] x86/hvm: Allow the guest to permit the use of userspace hypercalls
On 13/01/16 11:41, Stefano Stabellini wrote: > On Wed, 13 Jan 2016, Juergen Gross wrote: >> On 12/01/16 18:23, Stefano Stabellini wrote: >>> On Tue, 12 Jan 2016, Juergen Gross wrote: >>>> On 12/01/16 18:05, Stefano Stabellini wrote: >>>>> On Tue, 12 Jan 2016, Jan Beulich wrote: >>>>>>>>> On 12.01.16 at 13:07, <stefano.stabellini@xxxxxxxxxxxxx> wrote: >>>>>>> On Mon, 11 Jan 2016, David Vrabel wrote: >>>>>>>> On 11/01/16 17:17, Andrew Cooper wrote: >>>>>>>>> So from one point of view, sufficient justification for this change is >>>>>>>>> "because the Linux way isn't the only valid way to do this". >>>>>>>> >>>>>>>> "Because we can" isn't a good justification for adding something new. >>>>>>>> Particularly something that is trivially easy to (accidentally) misuse >>>>>>>> and open a big security hole between userspace and kernel. >>>>>>>> >>>>>>>> The vague idea for a userspace netfront that's floating around >>>>>>>> internally is also not a good reason for pushing this feature at this >>>>>>>> time. >>>>>>> >>>>>>> I agree with David, but I might have another good use case for this. >>>>>>> >>>>>>> Consider the following scenario: we have a Xen HVM guest, with Xen >>>>>>> installed inside of it (nested virtualization). I'll refer to Xen >>>>>>> running on the host as L0 Xen and Xen running inside the VM as L1 Xen. >>>>>>> Similarly we have two dom0 running, the one with access to the physical >>>>>>> hardware, L0 Dom0, and the one running inside the VM, L1 Dom0. >>>>>>> >>>>>>> Let's suppose that we want to lay the groundwork for L1 Dom0 to use PV >>>>>>> frontend drivers, netfront and blkfront to speed up execution. In order >>>>>>> to do that, the first thing it needs to do is making an hypercall to L0 >>>>>>> Xen. That's because netfront and blkfront needs to communicate with >>>>>>> netback and blkback in L0 Dom0: event channels and grant tables are the >>>>>>> ones provided by L0 Xen. >>>>>> >>>>>> That's again a layering violation (bypassing the L1 hypervisor). >>>>> >>>>> True, but in this scenario it might be necessary for performance >>>>> reasons: otherwise every hypercall would need to bounce off L1 Xen, >>>>> possibly cancelling the benefits of running netfront and blkfront in the >>>>> first place. I don't have numbers though. >>>> >>>> How is this supposed to work? How can dom0 make hypercalls to L1 _or_ L0 >>>> hypervisor? How can it select the hypervisor it is talking to? >>> >>> >From L0 Xen point of view, the guest is just a normal PV on HVM guest, >>> it doesn't matter what's inside, so L1 Dom0 is going to make hypercalls >>> to L0 Xen like any other PV on HVM guests: mapping the hypercall page by >>> writing to the right MSR, retrieved via cpuid, then calling into the >> >> But how to specify that cpuid/MSR should target the L0 hypervisor >> instead of L1? > > Keeping in mind that L1 Dom0 is a PV guest from L1 Xen point of view, > but a PV on HVM guest from L0 Xen point of view, it is true that the > cpuid could be an issue because the cpuid would be generated by L0 Xen, > but then would get filtered by L1 Xen. However the MSR should be OK, > assuming that L1 Xen allows access to it: from inside the VM it would > look like a regular machine MSR, it couldn't get confused with anything > causing hypercalls to L1 Xen. L1 Xen wouldn't allow access to it. Otherwise it couldn't ever setup a hypercall page for one of it's guests. >> And even if this would be working, just by mapping >> the correct page the instructions doing the transition to the >> hypervisor would still result in entering the L1 hypervisor, as >> those instructions must be handled by L1 first in order to make >> nested virtualization work. > > This is wrong. The hypercall page populated by L0 Xen would contain > vmcall instructions. When L1 Dom0 calls into the hypercall page, it > would end up making a vmcall, which brings it directly to L0 Xen, > skipping L1 Xen. Sure. And L0 Xen will see that this guest is subject to nested virtualization and is reflecting the vmcall to L1 Xen (see e.g. xen/arch/x86/hvm/svm/nestedsvm.c, nestedsvm_check_intercepts() ). How else would L1 Xen ever get a vmcall of one of it's guests? Juergen _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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