Re: API/ABIs: Re: [RFC PATCH v2 0/2] Add a new acquire resource to query vcpu statistics

[Jan Beulich <jbeulich@suse.com>]

On 06.03.2023 15:23, Matias Ezequiel Vara Larsen wrote:
> Regarding your email, I have the following comments:
> 
> - I still do not know how to choose the value of cacheline_size. I understand
> this value shall be between the actual cacheline_size and PAGE_SIZE. A value
> that could match most architectures could be 256 bytes.

This isn't a concern anymore when offset and stride are stored in the
header. It was a concern when trying to come up with a layout where
these value were to be inferred (or known a priori).

> - Xen shall use the "stats_active" field to indicate what it is producing. In
>   this field, reserved bits shall be 0. This shall allow us to agree on the
> layout even when producer and consumer are compiled with different headers.

I wonder how well such a bitfield is going to scale. It provides for
only 32 (maybe 64) counters. Of course this may seem a lot right now,
but you never know how quickly something like this can grow. Plus
with ...

> - In the vcpu_stats structure, new fields can only ever be appended.

... this rule the only ambiguity that could arise to consumers when
no active flags existed would be that they can't tell "event never
occurred" from "hypervisor doesn't know about this anymore".

Jan