On Fri, Feb 25, 2022, David Woodhouse wrote: > On Fri, 2022-02-25 at 16:13 +0000, Sean Christopherson wrote: > > On Fri, Feb 25, 2022, Woodhouse, David wrote: > > > Since we need an active vCPU context to do dirty logging (thanks, dirty > > > ring)... and since any time vcpu_run exits to userspace for any reason > > > might be the last time we ever get an active vCPU context... I think > > > that kind of fundamentally means that we must flush dirty state to the > > > log on *every* return to userspace, doesn't it? > > > > I would rather add a variant of mark_page_dirty_in_slot() that takes a vCPU, which > > we whould have in all cases. I see no reason to require use of kvm_get_running_vcpu(). > > We already have kvm_vcpu_mark_page_dirty(), but it can't use just 'some > vcpu' because the dirty ring is lockless. So if you're ever going to > use anything other than kvm_get_running_vcpu() we need to add locks. Heh, actually, scratch my previous comment. I was going to respond that kvm_get_running_vcpu() is mutually exclusive with all other ioctls() on the same vCPU by virtue of vcpu->mutex, but I had forgotten that kvm_get_running_vcpu() really should be "kvm_get_loaded_vcpu()". I.e. as long as KVM is in a vCPU-ioctl path, kvm_get_running_vcpu() will be non-null. > And while we *could* do that, I don't think it would negate the > fundamental observation that *any* time we return from vcpu_run to > userspace, that could be the last time. Userspace might read the dirty > log for the *last* time, and any internally-cached "oh, at some point > we need to mark <this> page dirty" is lost because by the time the vCPU > is finally destroyed, it's too late. Hmm, isn't that an existing bug? I think the correct fix would be to flush all dirty vmcs12 pages to the memslot in vmx_get_nested_state(). Userspace _must_ invoke that if it wants to migrated a nested vCPU. > I think I'm going to rip out the 'dirty' flag from the gfn_to_pfn_cache > completely and add a function (to be called with an active vCPU > context) which marks the page dirty *now*. Hrm, something like? 1. Drop @dirty from kvm_gfn_to_pfn_cache_init() 2. Rename @dirty => @old_dirty in kvm_gfn_to_pfn_cache_refresh() 3. Add an API to mark the associated slot dirty without unmapping I think that makes sense. > KVM_GUEST_USES_PFN users like nested VMX will be expected to do this > before returning from vcpu_run anytime it's in L2 guest mode. As above, I think the correct thing to do is enlightent the flows that retrieve the state being cached.
