Re: VMX and save/restore guest in virtual-8086 mode

[Avi Kivity <avi@redhat.com>]

On 04/07/2010 11:24 PM, Marcelo Tosatti wrote:
> 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);
>
>    


Looks like KVM_SET_REGS should write rmode.save_iopl (and a new save_vm)?

I think we have a small related bug in realmode emulation - we run the 
guest with iopl=3.  This means the guest can use pushfl and see the host 
iopl instead of the guest iopl.  We should run with iopl=0, which causes 
pushfl/popfl to #GP, where we can emulate the flags correctly (by 
updating rmode.save_iopl and rmode.save_vm).  That has lots of 
implications however...


> 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.
>    

Isn't it a very rare event?

> - Should be conditional on VMX !unrestricted guest.
>    

Userspace should know nothing of this mess.

> - Its a fugly workaround.
>    

True.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.