Re: [PATCH v7 10/10] x86/vm_event: Carry Processor Trace buffer offset in vm_event

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/02/2021 08:55, Jan Beulich wrote:
> On 30.01.2021 00:40, Tamas K Lengyel wrote:
>> On Fri, Jan 29, 2021 at 6:22 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
>> wrote:
>>> On 26/01/2021 14:27, Jan Beulich wrote:
>>>> On 21.01.2021 22:27, Andrew Cooper wrote:
>>>>> --- a/xen/arch/x86/vm_event.c
>>>>> +++ b/xen/arch/x86/vm_event.c
>>>>> @@ -251,6 +251,9 @@ void vm_event_fill_regs(vm_event_request_t *req)
>>>>>
>>>>>      req->data.regs.x86.shadow_gs = ctxt.shadow_gs;
>>>>>      req->data.regs.x86.dr6 = ctxt.dr6;
>>>>> +
>>>>> +    if ( hvm_vmtrace_output_position(curr, 
>>>>> &req->data.regs.x86.pt_offset) != 1 )
>>>>> +        req->data.regs.x86.pt_offset = ~0;
>>>> Ah. (Regarding my earlier question about this returning -errno or
>>>> boolean).
>>>>
>>>>> --- a/xen/include/public/vm_event.h
>>>>> +++ b/xen/include/public/vm_event.h
>>>>> @@ -223,6 +223,12 @@ struct vm_event_regs_x86 {
>>>>>       */
>>>>>      uint64_t npt_base;
>>>>>
>>>>> +    /*
>>>>> +     * Current offset in the Processor Trace buffer. For Intel Processor 
>>>>> Trace
>>>>> +     * this is MSR_RTIT_OUTPUT_MASK. Set to ~0 if no Processor Trace is 
>>>>> active.
>>>>> +     */
>>>>> +    uint64_t pt_offset;
>>>> According to vmtrace_output_position() the value is only one half
>>>> of what the named MSR contains. Perhaps "... this is from MSR_..."?
>>>> Not sure whether, despite this, there still is a reason to have
>>>> this 64-bit wide.
>>> This is a vestigial remnant which escaped the "use vmtrace uniformly" work.
>>>
>>> It should match the domctl_vmtrace_output_position() format, so each
>>> interface gives the same content for the attempted-platform-neutral version.
>> From the vm_event ABI perspective it's simpler to have a 64-bit value
>> here even if the max value it may possibly carry is never going to use
>> the whole 64-bit width. I rather not play with shortening it just to
>> add padding somewhere else.
>>
>> As for what it's called is not that important from my perspective,
>> vmtrace_pos or something like that for example is fine by me.
> The important thing to me is that the comment not be misleading.
> Whether that's arranged for by adjusting the comment of the
> commented declaration is up to what you deem better, i.e. I
> understand the comment it is.

Please see v8.  I rewrote the comment.

~Andrew