Sean, understood with many thanks! Good luck, Harry
On Wed, Jul 28, 2021 at 3:01 PM Sean Christopherson <sea...@google.com> wrote: > > On Wed, Jul 28, 2021, harry harry wrote: > > Sean, sorry for the late reply. Thanks for your careful explanations. > > > > > For emulation of any instruction/flow that starts with a guest virtual > > > address. > > > On Intel CPUs, that includes quite literally any "full" instruction > > > emulation, > > > since KVM needs to translate CS:RIP to a guest physical address in order > > > to fetch > > > the guest's code stream. KVM can't avoid "full" emulation unless the > > > guest is > > > heavily enlightened, e.g. to avoid string I/O, among many other things. > > > > Do you mean the emulated MMU is needed when it *only* wants to > > translate GVAs to GPAs in the guest level? > > Not quite, though gva_to_gpa() is the main use. The emulated MMU is also > used to > inject guest #PF and to load/store guest PDTPRs. > > > In such cases, the hardware MMU cannot be used because hardware MMU > > can only translate GVAs to HPAs, right? > > Sort of. The hardware MMU does translate GVA to GPA, but the GPA value is not > visible to software (unless the GPA->HPA translation faults). That's also > true > for VA to PA (and GVA to HPA). Irrespective of virtualization, x86 ISA > doesn't > provide an instruction to retrive the PA for a given VA. > > If such an instruction did exist, and it was to be usable for a VMM to do a > GVA->GPA translation, the magic instruction would need to take all MMU params > as > operands, e.g. CR0, CR3, CR4, and EFER. When KVM is active (not the guest), > the > hardware MMU is loaded with the host MMU configuration, not the guest. In > both > VMX and SVM, vCPU state is mostly ephemeral in the sense that it ceases to > exist > in hardware when the vCPU exits to the host. Some state is retained in > hardware, > e.g. TLB and cache entries, but those are associated with select properties of > the vCPU, e.g. EPTP, CR3, etc..., not with the vCPU itself, i.e. not with the > VMCS (VMX) / VMCB (SVM).
