I don’t think that the bitmap by itself is really a performance bottleneck here. Thanks, Ashish > On Dec 7, 2020, at 9:10 PM, Steve Rutherford <srutherford@xxxxxxxxxx> wrote: > > On Mon, Dec 7, 2020 at 12:42 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: >> >>> On Sun, Dec 06, 2020, Paolo Bonzini wrote: >>> On 03/12/20 01:34, Sean Christopherson wrote: >>>> On Tue, Dec 01, 2020, Ashish Kalra wrote: >>>>> From: Brijesh Singh <brijesh.singh@xxxxxxx> >>>>> >>>>> KVM hypercall framework relies on alternative framework to patch the >>>>> VMCALL -> VMMCALL on AMD platform. If a hypercall is made before >>>>> apply_alternative() is called then it defaults to VMCALL. The approach >>>>> works fine on non SEV guest. A VMCALL would causes #UD, and hypervisor >>>>> will be able to decode the instruction and do the right things. But >>>>> when SEV is active, guest memory is encrypted with guest key and >>>>> hypervisor will not be able to decode the instruction bytes. >>>>> >>>>> Add SEV specific hypercall3, it unconditionally uses VMMCALL. The hypercall >>>>> will be used by the SEV guest to notify encrypted pages to the hypervisor. >>>> >>>> What if we invert KVM_HYPERCALL and X86_FEATURE_VMMCALL to default to VMMCALL >>>> and opt into VMCALL? It's a synthetic feature flag either way, and I don't >>>> think there are any existing KVM hypercalls that happen before alternatives are >>>> patched, i.e. it'll be a nop for sane kernel builds. >>>> >>>> I'm also skeptical that a KVM specific hypercall is the right approach for the >>>> encryption behavior, but I'll take that up in the patches later in the series. >>> >>> Do you think that it's the guest that should "donate" memory for the bitmap >>> instead? >> >> No. Two things I'd like to explore: >> >> 1. Making the hypercall to announce/request private vs. shared common across >> hypervisors (KVM, Hyper-V, VMware, etc...) and technologies (SEV-* and TDX). >> I'm concerned that we'll end up with multiple hypercalls that do more or >> less the same thing, e.g. KVM+SEV, Hyper-V+SEV, TDX, etc... Maybe it's a >> pipe dream, but I'd like to at least explore options before shoving in KVM- >> only hypercalls. >> >> >> 2. Tracking shared memory via a list of ranges instead of a using bitmap to >> track all of guest memory. For most use cases, the vast majority of guest >> memory will be private, most ranges will be 2mb+, and conversions between >> private and shared will be uncommon events, i.e. the overhead to walk and >> split/merge list entries is hopefully not a big concern. I suspect a list >> would consume far less memory, hopefully without impacting performance. > > For a fancier data structure, I'd suggest an interval tree. Linux > already has an rbtree-based interval tree implementation, which would > likely work, and would probably assuage any performance concerns. > > Something like this would not be worth doing unless most of the shared > pages were physically contiguous. A sample Ubuntu 20.04 VM on GCP had > 60ish discontiguous shared regions. This is by no means a thorough > search, but it's suggestive. If this is typical, then the bitmap would > be far less efficient than most any interval-based data structure. > > You'd have to allow userspace to upper bound the number of intervals > (similar to the maximum bitmap size), to prevent host OOMs due to > malicious guests. There's something nice about the guest donating > memory for this, since that would eliminate the OOM risk.
