Re: [PATCH] hyperv: root partition faults writing to VP ASSIST MSR PAGE

[Praveen Kumar <kumarpraveen@linux.microsoft.com>]

On 21-07-2021 15:40, Wei Liu wrote:
> On Wed, Jul 21, 2021 at 12:42:52PM +0530, Praveen Kumar wrote:
>> On 21-07-2021 09:40, Michael Kelley wrote:
>>> From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, July 20, 2021 9:29 AM
>>>>
>>>> On Tue, Jul 20, 2021 at 04:20:44PM +0000, Michael Kelley wrote:
>>>>> From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, July 20, 2021 6:35 AM
>>>>>>
>>>>>> On Tue, Jul 20, 2021 at 06:55:56PM +0530, Praveen Kumar wrote:
>>>>>> [...]
>>>>>>>>
>>>>>>>>> +	if (hv_root_partition &&
>>>>>>>>> +	    ms_hyperv.features & HV_MSR_APIC_ACCESS_AVAILABLE) {
>>>>>>>>
>>>>>>>> Is HV_MSR_APIC_ACCESS_AVAILABLE a root only flag? Shouldn't non-root
>>>>>>>> kernel check this too?
>>>>>>>
>>>>>>> Yes, you are right. Will update this in v2. thanks.
>>>>>>
>>>>>> Please split adding this check to its own patch.
>>>>>>
>>>>>> Ideally one patch only does one thing.
>>>>>>
>>>>>> Wei.
>>>>>>
>>>>>
>>>>> I was just looking around in the Hyper-V TLFS, and I didn't see
>>>>> anywhere that the ability to set up a VP Assist page is dependent
>>>>> on HV_MSR_APIC_ACCESS_AVAILABLE.  Or did I just miss it?
>>>>
>>>> The feature bit Praveen used is wrong and should be fixed.
>>>>
>>>> Per internal discussion this is gated by the AccessIntrCtrlRegs bit.
>>>>
>>>> Wei.
>>>>
>>>
>>> The AccessIntrCtrlRegs bit *is* HV_MSR_APIC_ACCESS_AVAILABLE.
>>> Both are defined as bit 4 of the Partition Privilege flags.  :-)   I don't
>>> know why the names don't line up.   Even so, it's not clear to me that
>>> AccessIntrCtrlRegs has any bearing on the VP Assist page.  I see this
>>> description of AccessIntrCtrlRegs:
>>>
>>
>> Yup, what I understood as well, this is the one required one for Partition Privilege Flags (4th bit), however, cannot comment on the naming convention.
>>
>>      5 /* Virtual APIC assist and VP assist page registers available */
>>      4 #define HV_MSR_APIC_ACCESS_AVAILABLE            BIT(4)
>>
> 
> Urgh, okay. It is my fault for not reading the code closely. Sorry for
> the confusion.
> 
>>> The partition has access to the synthetic MSRs associated with the
>>> APIC (HV_X64_MSR_EOI, HV_X64_MSR_ICR and HV_X64_MSR_TPR).
>>> If this flag is cleared, accesses to these MSRs results in a #GP fault if
>>> the MSR intercept is not installed.
>>>
>>
>> As per what I also understood from the TLFS doc,that we let partition
>> access the MSR and do a fault.  However, the point is, does it make
>> sense to allocate page for vp assist and perform action which is meant
>> to fail when the flag is cleared ?
> 
> Like Michael said, there are some other things that are not tied to that
> particular bit. We should get more clarity on what gates what.  Perhaps
> that privilege bit only controls access to the EOI assist bit and the
> other things in the VP assist page are gated by other privilege bits.
> This basically means we should setup the page when there is at least one
> thing in that page can be used.
> 
> This is mostly an orthogonal issue from the one we want to fix. In
> the interest of making progress we can drop the new check for now and
> just add a root specific path for setting up and tearing down the VP
> assist pages.
> 
> How does that sound?
> 

Sounds good to me. Thanks Wei.

> Wei.
> 
>>
>>> But maybe you have additional info that applies to the root
>>> partition that is not in the TLFS.
>>>
>>
>> As per what discussed internally and I understood, the root partition
>> shares the vp assist page provided by hypervisor and its read only for
>> Root kernel.
>>
>>> Michael

Regards,

~Praveen.