Re: [Qemu-devel] [PATCH 4/7 v2] KVM regsync: Add register bitmap parameter to do_kvm_cpu_synchronize_state

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Jan 16, 2013 at 05:00:52PM +0000, Bhushan Bharat-R65777 wrote:
> Second-)
> Currently kvm_arch_get_registers() is not optimized in two sense; one, it always get all registers from KVM; two, in kvm_arch_get_registers() it copies all registers to env->. This patch-set handles the second issue of optimization, copy only the requested registers to env-> in kvm_arch_get_registers(), plus when kvm_arch_put_registers() is called then it copies only the modified registers for KVM_SET_SREGS.
> 
> This optimization is looking good to me and allows sync of registers via one common kvm_arch_get/set_registers() and no separate function definition for setting is needed for timer related changes.

To be clearer. For example, executing:

- kvm_get_regs(FULL_STATE)
- kvm_put_regs(FULL_STATE)

Is reading and writing the full register set while
in runtime state.

/* full state set, modified during initialization or on vmload */
#define KVM_PUT_FULL_STATE      3

Again, its necessary to read the full state on "cpu_synchronize_state" 
because its not specified which registers can be modified
during ioctl(KVM_RUN) (therefore separating get registers operation
in full/reset/runtime sets is awkward).

But direct read/write of registers is also bad: if a given register is
read and modified (but not written back), then reading through ioctl()
returns a wrong value.

Apparently accessors are the best option.