Re: [PATCH v2 4/7] x86/vmx: add do_vmtrace_op

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.06.2020 18:02, Michał Leszczyński wrote:
> ----- 22 cze 2020 o 17:22, Jan Beulich jbeulich@xxxxxxxx napisał(a):
>> On 22.06.2020 16:35, Michał Leszczyński wrote:
>>> ----- 22 cze 2020 o 15:25, Jan Beulich jbeulich@xxxxxxxx napisał(a):
>>>> Is any of what you do in this switch() actually legitimate without
>>>> hvm_set_vmtrace_pt_size() having got called for the guest? From
>>>> remarks elsewhere I imply you expect the param that you currently
>>>> use to be set upon domain creation time, but at the very least the
>>>> potentially big buffer should imo not get allocated up front, but
>>>> only when tracing is to actually be enabled.
>>>
>>> Wait... so you want to allocate these buffers in runtime?
>>> Previously we were talking that there is too much runtime logic
>>> and these enable/disable hypercalls should be stripped to absolute
>>> minimum.
>>
>> Basic arrangements can be made at domain creation time. I don't
>> think though that it would be a good use of memory if you
>> allocated perhaps many gigabytes of memory just for possibly
>> wanting to enable tracing on a guest.
>>
> 
> From our previous conversations I thought that you want to have
> as much logic moved to the domain creation as possible.
> 
> Thus, a parameter "vmtrace_pt_size" was introduced. By default it's
> zero (= disabled), if you set it to a non-zero value, then trace buffers
> of given size will be allocated for the domain and you have possibility
> to use ipt_enable/ipt_disable at any moment.
> 
> This way the runtime logic is as thin as possible. I assume user knows
> in advance whether he/she would want to use external monitoring with IPT
> or not.

Andrew - I think you requested movement to domain_create(). Could
you clarify if indeed you mean to also allocate the big buffers
this early?

> It's also not "many gigabytes". In most use cases a buffer of 16/32/64 MB
> would suffice, I think.

But that one such buffer per vCPU, isn't it? Plus these buffers
need to be physically contiguous, which is an additional possibly
severe constraint.

Jan