VMX and save/restore guest in virtual-8086 mode

[Marcelo Tosatti <mtosatti@redhat.com>]

During initialization, WinXP.32 switches to virtual-8086 mode, with
paging enabled, to use VGABIOS functions.

Since enter_pmode unconditionally clears IOPL and VM bits in RFLAGS

        flags = vmcs_readl(GUEST_RFLAGS);
        flags &= ~(X86_EFLAGS_IOPL | X86_EFLAGS_VM);
        flags |= (vmx->rmode.save_iopl << IOPL_SHIFT);
        vmcs_writel(GUEST_RFLAGS, flags);

And the order of loading state is set_regs (rflags) followed by
set_sregs (cr0), these bits are lost across save/restore:

savevm 1
kvm_arch_save_regs EIP=7a04 cr0=8001003b eflags=33286
system_reset
loadvm 1
kvm_arch_save_regs EIP=7a04 cr0=8001003b eflags=10286
cont
kvm: unhandled exit 80000021
kvm_run returned -22

The following patch fixes it, but it has some drawbacks:

- cpu_synchronize_state+writeback is noticeably slow with tpr patching,
  this makes it slower.
- Should be conditional on VMX !unrestricted guest.
- Its a fugly workaround.

Any better ideas?

diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
index 748ff69..9821653 100644
--- a/qemu-kvm-x86.c
+++ b/qemu-kvm-x86.c
@@ -956,6 +956,7 @@ void kvm_arch_load_regs(CPUState *env, int level)
     sregs.efer = env->efer;
 
     kvm_set_sregs(env, &sregs);
+    kvm_set_regs(env, &regs);
 
     /* msrs */
     n = 0;