Re: [PATCH 2/4] kvm: svm: Enable UMIP feature on AMD

["Moger, Babu" <Babu.Moger@amd.com>]

On 11/1/19 2:24 PM, Andy Lutomirski wrote:
> On Fri, Nov 1, 2019 at 12:20 PM Moger, Babu <Babu.Moger@amd.com> wrote:
>>
>>
>>
>> On 11/1/19 1:29 PM, Jim Mattson wrote:
>>> On Fri, Nov 1, 2019 at 10:33 AM Moger, Babu <Babu.Moger@amd.com> wrote:
>>>>
>>>> AMD 2nd generation EPYC processors support UMIP (User-Mode Instruction
>>>> Prevention) feature. The UMIP feature prevents the execution of certain
>>>> instructions if the Current Privilege Level (CPL) is greater than 0.
>>>> If any of these instructions are executed with CPL > 0 and UMIP
>>>> is enabled, then kernel reports a #GP exception.
>>>>
>>>> The idea is taken from articles:
>>>> https://lwn.net/Articles/738209/
>>>> https://lwn.net/Articles/694385/
>>>>
>>>> Enable the feature if supported on bare metal and emulate instructions
>>>> to return dummy values for certain cases.
>>>>
>>>> Signed-off-by: Babu Moger <babu.moger@amd.com>
>>>> ---
>>>>  arch/x86/kvm/svm.c |   21 ++++++++++++++++-----
>>>>  1 file changed, 16 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>>> index 4153ca8cddb7..79abbdeca148 100644
>>>> --- a/arch/x86/kvm/svm.c
>>>> +++ b/arch/x86/kvm/svm.c
>>>> @@ -2533,6 +2533,11 @@ static void svm_decache_cr4_guest_bits(struct kvm_vcpu *vcpu)
>>>>  {
>>>>  }
>>>>
>>>> +static bool svm_umip_emulated(void)
>>>> +{
>>>> +       return boot_cpu_has(X86_FEATURE_UMIP);
>>>> +}
>>>
>>> This makes no sense to me. If the hardware actually supports UMIP,
>>> then it doesn't have to be emulated.
>> My understanding..
>>
>> If the hardware supports the UMIP, it will generate the #GP fault when
>> these instructions are executed at CPL > 0. Purpose of the emulation is to
>> trap the GP and return a dummy value. Seems like this required in certain
>> legacy OSes running in protected and virtual-8086 modes. In long mode no
>> need to emulate. Here is the bit explanation https://lwn.net/Articles/738209/
>>
> 
> Indeed.  Again, what does this have to do with your patch?
> 
>>
>>>
>>> To the extent that kvm emulates UMIP on Intel CPUs without hardware
>>> UMIP (i.e. smsw is still allowed at CPL>0), we can always do the same
>>> emulation on AMD, because SVM has always offered intercepts of sgdt,
>>> sidt, sldt, and str. So, if you really want to offer this emulation on
>>> pre-EPYC 2 CPUs, this function should just return true. But, I have to
>>> ask, "why?"
>>
>>
>> Trying to support UMIP feature only on EPYC 2 hardware. No intention to
>> support pre-EPYC 2.
>>
> 
> I think you need to totally rewrite your changelog to explain what you
> are doing.
> 
> As I understand it, there are a couple of things KVM can do:
> 
> 1. If the underlying hardware supports UMIP, KVM can expose UMIP to
> the guest.  SEV should be irrelevant here.
> 
> 2. Regardless of whether the underlying hardware supports UMIP, KVM
> can try to emulate UMIP in the guest.  This may be impossible if SEV
> is enabled.
> 
> Which of these are you doing?
> 
My intention was to do 1.  Let me go back and think about this again.