Re: [PATCH v2 2/4] x86: Introduce MSR_UNHANDLED

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.02.2021 13:17, Roger Pau Monné wrote:
> On Tue, Feb 23, 2021 at 11:15:31AM +0100, Jan Beulich wrote:
>> On 23.02.2021 10:34, Roger Pau Monné wrote:
>>> On Tue, Feb 23, 2021 at 08:57:00AM +0100, Jan Beulich wrote:
>>>> On 22.02.2021 22:19, Boris Ostrovsky wrote:
>>>>>
>>>>> On 2/22/21 6:08 AM, Roger Pau Monné wrote:
>>>>>> On Fri, Feb 19, 2021 at 09:56:32AM -0500, Boris Ostrovsky wrote:
>>>>>>> On 2/18/21 5:51 AM, Roger Pau Monné wrote:
>>>>>>>> On Wed, Jan 20, 2021 at 05:49:10PM -0500, Boris Ostrovsky wrote:
>>>>>>>>> When toolstack updates MSR policy, this MSR offset (which is the last
>>>>>>>>> index in the hypervisor MSR range) is used to indicate hypervisor
>>>>>>>>> behavior when guest accesses an MSR which is not explicitly emulated.
>>>>>>>> It's kind of weird to use an MSR to store this. I assume this is done
>>>>>>>> for migration reasons?
>>>>>>>
>>>>>>> Not really. It just seemed to me that MSR policy is the logical place 
>>>>>>> to do that. Because it *is* a policy of how to deal with such accesses, 
>>>>>>> isn't it?
>>>>>> I agree that using the msr_policy seems like the most suitable place
>>>>>> to convey this information between the toolstack and Xen. I wonder if
>>>>>> it would be fine to have fields in msr_policy that don't directly
>>>>>> translate into an MSR value?
>>>>>
>>>>>
>>>>> We have xen_msr_entry_t.flags that we can use when passing policy array 
>>>>> back and forth. Then we can ignore xen_msr_entry_t.idx for that entry 
>>>>> (although in earlier version of this series Jan preferred to use idx and 
>>>>> leave flags alone).
>>>>
>>>> Which, just to clarify, was not the least because of the flags
>>>> field being per-entry, i.e. per MSR, while here we want a global
>>>> indicator.
>>>
>>> We could exploit a bit the xen_msr_entry_t structure and use it
>>> like:
>>>
>>> typedef struct xen_msr_entry {
>>>     uint32_t idx;
>>> #define XEN_MSR_IGNORE (1u << 0)
>>>     uint32_t flags;
>>>     uint64_t val;
>>> } xen_msr_entry_t;
>>>
>>> Then use the idx and val fields to signal the start and size of the
>>> range to ignore accesses to when XEN_MSR_IGNORE is set in the flags
>>> field?
>>>
>>> xen_msr_entry_t = {
>>>     .idx = 0,
>>>     .val = 0xffffffff,
>>>     .flags = XEN_MSR_IGNORE,
>>> };
>>>
>>> Would be equivalent to ignoring accesses to the whole MSR range.
>>>
>>> This would allow selectively selecting which MSRs to ignore, while
>>> not wasting a MSR itself to convey the information?
>>
>> Hmm, yes, the added flexibility would be nice from an abstract pov
>> (not sure how relevant it would be to Solaris'es issue). But my
>> dislike of using a flag which is meaningless in ordinary entries
>> remains, as was voiced against Boris'es original version.
> 
> I understand the flags field is meaningless for regular MSRs, but I
> don't see why it would be an issue to start using it for this specific
> case of registering ranges of ignored MSRs.

It's not an "issue", it is - as said - my dislike. However, the way
it is in your proposal it is perhaps indeed not as bad as in Boris'es
original one: The flag now designates the purpose of the entry, and
the other two fields still have a meaning. Hence I was wrong to state
that it's "meaningless" - it now is required to be clear for
"ordinary" entries.

In principle there could then also be multiple such entries / ranges.

> It certainly seems better than hijacking an MSR index (MSR_UNHANDLED).

Not sure.

Jan