On 02/16/2012 04:46 PM, Anthony Liguori wrote: >> What will it buy us? Surely not speed. Entering a guest is not much >> (if at all) faster than exiting to userspace and any non trivial >> operation will require exit to userspace anyway, > > > You can emulate the PIT/RTC entirely within the guest using kvmclock > which doesn't require an additional exit to get the current time base. > > So instead of: > > 1) guest -> host kernel > 2) host kernel -> userspace > 3) implement logic using rdtscp via VDSO > 4) userspace -> host kernel > 5) host kernel -> guest > > You go: > > 1) guest -> host kernel > 2) host kernel -> guest (with special CR3) > 3) implement logic using rdtscp + kvmclock page > 4) change CR3 within guest and RETI to VMEXIT source RIP > > Same basic concept as PS/2 emulation with SMM.
Interesting, but unimplementable in practice. SMM requires a VMEXIT for RSM, and anything non-SMM wants a virtual address mapping (and some RAM) which you can't get without guest cooperation. There are other complications like an NMI interrupting hypervisor-provided code and finding unexpected addresses on its stack (SMM at least blocks NMIs). Tangentially related, Intel introduced a VMFUNC that allows you to change the guest's physical memory map to a pre-set alternative provided by the host, without a VMEXIT. Seems similar to SMM but requires guest cooperation. I guess it's for unintrusive virus scanners and the like. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
