Re: [Xen-devel] [PATCH v3] arch/x86: Add registers to vm_event

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 10/25/18 4:10 PM, Alexandru Stefan ISAILA wrote:
> On 25.10.2018 14:55, Jan Beulich wrote:
>>>>> On 18.10.18 at 11:02, <aisaila@xxxxxxxxxxxxxxx> wrote:
>>> +struct x86_selector_reg {
>>> +    union
>>> +    {
>>> +        uint64_t bytes;
>>> +        struct
>>> +        {
>>> +            uint32_t base;
>>> +            uint32_t limit  :    20;
>>> +            uint32_t ar     :    12;
>>> +        } fields;
>>> +    };
>>> +};
>>
>> I don't understand why sel was moved out. Are you tight on
>> space here, such that you can't tolerate the padding bytes?
> 
> It was dropped on Andrew's suggestion. We are ok with it in the 
> structure so if is ok by you I can add it back.

We are tight on space. The ring buffer is only 4K, and it already can
only fit about 10 sync vm_events before VCPUs need to be blocked waiting
for a place in the buffer. That's why we've attempted the packing.

>> Finally - what meaning to the low (or high) 4 bits of "ar"
>> carry?
> 
> If I correctly understand the question, we use ar bits to  determine the 
> running mode of the guest.

That's what we need the CS .ar for: looking at CS_AR_BYTES_D and
CS_AR_BYTES_L, the rest we pass along to the introspection engine - I'd
have to check with them for more details if requested.

But I think we've probably misunderstood the question.


Thanks,
Razvan

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