Re: [PATCH 3/7 v7] KVM: nSVM: No need to set bits 11:0 in MSRPM and IOPM bitmaps

[Krish Sadhukhan <krish.sadhukhan@oracle.com>]

On 4/20/21 1:00 PM, Sean Christopherson wrote:
> On Mon, Apr 12, 2021, Krish Sadhukhan wrote:
>> According to APM vol 2, hardware ignores the low 12 bits in MSRPM and IOPM
>> bitmaps. Therefore setting/unssetting these bits has no effect as far as
>> VMRUN is concerned. Also, setting/unsetting these bits prevents tests from
>> verifying hardware behavior.
>>
>> Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
>> ---
>>   arch/x86/kvm/svm/nested.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
>> index ae53ae46ebca..fd42c8b7f99a 100644
>> --- a/arch/x86/kvm/svm/nested.c
>> +++ b/arch/x86/kvm/svm/nested.c
>> @@ -287,8 +287,6 @@ static void nested_load_control_from_vmcb12(struct vcpu_svm *svm,
>>   
>>   	/* Copy it here because nested_svm_check_controls will check it.  */
>>   	svm->nested.ctl.asid           = control->asid;
>> -	svm->nested.ctl.msrpm_base_pa &= ~0x0fffULL;
>> -	svm->nested.ctl.iopm_base_pa  &= ~0x0fffULL;
> This will break nested_svm_vmrun_msrpm() if L1 passes an unaligned address.
> The shortlog is also wrong, KVM isn't setting bits, it's clearing bits.
>
> I also don't think svm->nested.ctl.msrpm_base_pa makes its way to hardware; IIUC,
> it's a copy of vmcs12->control.msrpm_base_pa.  The bitmap that gets loaded into
> the "real" VMCB is vmcb02->control.msrpm_base_pa.


Not sure if there's a problem with my patch as such, but upon inspecting 
the code, I see something missing:

     In nested_load_control_from_vmcb12(), we are not really loading 
msrpm_base_pa from vmcb12 even     though the name of the function 
suggests so.

     Then nested_vmcb_check_controls() checks msrpm_base_pa from 
'nested.ctl' which doesn't have         the copy from vmcb12.

     Then nested_vmcb02_prepare_control() prepares the vmcb02 copy of 
msrpm_base_pa from vmcb01.ptr->control.msrpm_base_pa.

     Then nested_svm_vmrun_msrpm() uses msrpm_base_pa from 'nested.ctl'.


Aren't we actually using msrpm_base_pa from vmcb01 instead of vmcb02 ?

>>   }
>>   
>>   /*
>> -- 
>> 2.27.0
>>