Re: [Qemu-devel] [RFC] Next gen kvm api

[Avi Kivity <avi@redhat.com>]

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.