On Thu, Aug 25, 2022, Mingwei Zhang wrote: > On Thu, Aug 25, 2022 at 7:41 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > On Wed, Aug 24, 2022, Jim Mattson wrote: > > > On Wed, Aug 24, 2022 at 5:11 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > > > > @google folks, what would it take for us to mark KVM_REQ_GET_NESTED_STATE_PAGES > > > > as deprecated in upstream and stop accepting patches/fixes? IIUC, when we eventually > > > > move to userfaultfd, all this goes away, i.e. we do want to ditch this at some point. > > > > > > Userfaultfd is a red herring. There were two reasons that we needed > > > this when nested live migration was implemented: > > > 1) our netlink socket mechanism for funneling remote page requests to > > > a userspace listener was broken. > > > 2) we were not necessarily prepared to deal with remote page requests > > > during VM setup. > > > > > > (1) has long since been fixed. Though our preference is to exit from > > > KVM_RUN and get the vCPU thread to request the remote page itself, we > > > are now capable of queuing a remote page request with a separate > > > listener thread and blocking in the kernel until the page is received. > > > I believe that mechanism is functionally equivalent to userfaultfd, > > > though not as elegant. > > > I don't know about (2). I'm not sure when the listener thread is set > > > up, relative to all of the other setup steps. Eliminating > > > KVM_REQ_GET_NESTED_STATE_PAGES means that userspace must be prepared > > > to fetch a remote page by the first call to KVM_SET_NESTED_STATE. The > > > same is true when using userfaultfd. > > > > > > These new ordering constraints represent a UAPI breakage, but we don't > > > seem to be as concerned about that as we once were. Maybe that's a > > > good thing. Can we get rid of all of the superseded ioctls, like > > > KVM_SET_CPUID, while we're at it? > > > > I view KVM_REQ_GET_NESTED_STATE_PAGES as a special case. We are likely the only > > users, we can (eventually) wean ourselves off the feature, and we can carry > > internal patches (which we are obviously already carrying) until we transition > > away. And unlike KVM_SET_CPUID and other ancient ioctls() that are largely > > forgotten, this feature is likely to be a maintenance burden as long as it exists. > > KVM_REQ_GET_NESTED_STATE_PAGES has been uniformly used in > KVM_SET_NESTED_STATE ioctl in VMX (including eVMCS) and SVM, it is > basically a two-step setting up of a nested state mechanism. > > We can change that, but this may have side effects and I think this > usage case has nothing to do with demand paging. There are two uses of KVM_REQ_GET_NESTED_STATE_PAGES: 1. Defer loads when leaving SMM. 2: Defer loads for KVM_SET_NESTED_STATE. #1 is fully solvable without a request, e.g. split ->leave_smm() into two helpers, one that restores whatever metadata is needed before restoring from SMRAM, and a second to load guest virtualization state that _after_ restoring all other guest state from SMRAM. #2 is done because of the reasons Jim listed above, which are specific to demand paging (including userfaultfd). There might be some interactions with other ioctls() (KVM_SET_SREGS?) that are papered over by the request, but that can be solved without a full request since only the first KVM_RUN after KVM_SET_NESTED_STATE needs to refresh things (though ideally we'd avoid that). In other words, if the demand paging use case goes away, then KVM can get rid of KVM_REQ_GET_NESTED_STATE_PAGES. > KVM_SET_NESTED_STATE in VMX, while in SVM implementation, it is simply > just a kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu); svm_set_nested_state() very rougly open codes enter_svm_guest_mode(). VMX could do the same, but that may or may not be a net positive. > hmm... so is the nested_vmx_enter_non_root_mode() call in vmx > KVM_SET_NESTED_STATE ioctl() still necessary? I am thinking that > because the same function is called again in nested_vmx_run(). nested_vmx_run() is used only to emulate VMLAUNCH/VMRESUME and wont' be invoked if the vCPU is already running L2 at the time of migration.
