On 9/8/2025 1:06 PM, Nuno Das Neves wrote:
On 9/8/2025 10:22 AM, Easwar Hariharan wrote:
On 9/8/2025 10:04 AM, Nuno Das Neves wrote:
On 9/5/2025 12:21 PM, Easwar Hariharan wrote:
On 8/28/2025 5:43 PM, Nuno Das Neves wrote:
From: Purna Pavan Chandra Aekkaladevi <paekkalad...@linux.microsoft.com>
Some versions of the hypervisor do not support HV_STATUS_AREA_PARENT and
return HV_STATUS_INVALID_PARAMETER for the second stats page mapping
request.
This results a failure in module init. Instead of failing, gracefully
fall back to populating stats_pages[HV_STATS_AREA_PARENT] with the
already-mapped stats_pages[HV_STATS_AREA_SELF].
What's the impact of this graceful fallback? It occurs to me that if a stats
accumulator, in userspace perhaps, expected to get stats from the 2 pages,
it'd get incorrect values.
This is going out of scope of this series a bit but I'll explain briefly.
When we do add the code to expose these stats to userspace, the SELF and
PARENT pages won't be exposed separately, there is no duplication.
For each stat counter in the page, we'll expose either the SELF or PARENT
value, depending on whether there is anything in that slot (whether it's zero
or not).
Some stats are available via the SELF page, and some via the PARENT page, but
the counters in the page have the same layout. So some counters in the SELF
page will all stay zero while on the PARENT page they are updated, and vice
versa.
I believe the hypervisor takes this strange approach for the purpose of
backward compatibility. Introducing L1VH created the need for this SELF/PARENT
distinction.
Hope that makes some kind of sense...it will be clearer when we post the mshv
debugfs code itself.
To put it another way, falling back to the SELF page won't cause any impact
to userspace because the distinction between the pages is all handled in the
driver, and we only read each stat value from either SELF or PARENT.
Nuno
Thank you for that explanation, it sorta makes sense.
I think it'd be better if this patch is part of the series that exposes the
stats
to userspace, so that it can be reviewed in context with the rest of the code in
the driver that manages the pick-and-choose of a stat value from the SELF/PARENT
page.
Good idea, I think I'll do that. Thanks!
Unless there's an active problem now in the upstream kernel that this patch
solves?
i.e. are the versions of the hypervisor that don't support the PARENT stats
page available in the wild?
I thought there was, but on reflection, no it doesn't solve a problem that
exists in
the code today.
Nuno
The usecases for stats exposed by the hypervisor are:
1) used within the kernel by root scheduler
2) exposed to userspace via debugfs.
I thought we are addressing the first use-case here (patch1 in this
series). If root scheduler support was upstreamed then this patchset
does solve a problem in upstream code.
Thanks,
Easwar (he/him)
--
Regards,
Praveen K Paladugu
