Re: [PATCH v3 4/4] KVM: x86 emulator: Allow PM/VM86 switch during task switch

[Kevin Wolf <kwolf@redhat.com>]

Am 06.02.2012 11:32, schrieb Avi Kivity:
> On 02/03/2012 08:29 PM, Kevin Wolf wrote:
>> Task switches can switch between Protected Mode and VM86. The current
>> mode must be updated during the task switch emulation so that the new
>> segment selectors are interpreted correctly.
>>
>> In order to let privilege checks succeed, rflags needs to be updated in
>> the vcpu struct as this causes a CPL update.
>>
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 4124a7e..38d5ef3 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -1286,6 +1286,7 @@ static unsigned long svm_get_rflags(struct kvm_vcpu *vcpu)
>>  static void svm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>>  {
>>  	to_svm(vcpu)->vmcb->save.rflags = rflags;
>> +	svm_update_cpl(vcpu);
>>  }
> 
> This can trigger the (cs&3)!=0 && (cr0&1) != 0 problem, right after we
> enter protected mode.
> 
> What this code does is slave the cpl to other x86 state (copying vmx),
> whereas in reality it is separate state (with independent existence from
> the mov cr0 instruction until the next far jump...).

Yes, as discussed in v2, eventually we'll want to call set_cpl() rather
than set_rflags() during the task switch (I left a TODO comment there).
Then the not quite correct update in svm_set_rflags can be removed again.

> We can fix this with
> 
>     if ((save.rflags ^ rflags) & EFLAGS_VM)
>         svm_update_cpl(vcpu)
> 
> but that leaves us with an unupdated cpl after live migration (well, it
> will update after loading segment registers).  Both are bad, but I think
> my version is less bad - live migration is broken anyway in this window,
> since we don't save the cpl.

Right, it just leaves broken what is already broken. If the rest of the
series is okay now, I'll resend with a check if rflags.VM has changed.

Kevin