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

[Jan Kiszka <jan.kiszka@siemens.com>]

Avi Kivity wrote:
> 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)?

Just like we manipulate the flags for guest debugging in the
set/get_rflags vendor handlers, the same should happen for IOPL and VM.
This is no business of enter_pmode/rmode.

> 
> 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?

It has to be - otherwise the decision to go for full sync and individual
get/set IOCTL would have been wrong. What happens during tpr patching?

> 
>> - Should be conditional on VMX !unrestricted guest.
>>    
> 
> Userspace should know nothing of this mess.
> 
>> - Its a fugly workaround.
>>    
> 
> True.
> 

Still likely the way to go for old kernels.

Jan

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