[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 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? Juergen _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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