Re: [PATCH 5/5] qmp: Added qemu-ebpf-rss-path command.

[Markus Armbruster]

Yuri Benditovich <yuri.benditovich@daynix.com> writes:

> On Tue, Aug 24, 2021 at 9:41 AM Markus Armbruster <armbru@redhat.com> wrote:
>>
>> Andrew Melnichenko <andrew@daynix.com> writes:
>>
>> > Hi,
>> >
>> >> The helper may or may not be installed at the path compiled into QEMU.
>> >>
>> > Yes, so the helper will not be called - QEMU will try to initiate eBPF RSS
>> > or use "in-qemu" RSS.
>>
>> My point is: the proposed command's mission is to help the management
>> application run the right helper.  However, its advice is *unreliable*.
>> It may point to the wrong helper, or to nothing at all.  The right
>> helper may still exist elsewhere.
>
> Hi Markus,
> Indeed the intention of this command is to return the proper helper.
> Especially in the case of RSS helper this is *reliable* advice and it
> points to the helper that was built together with QEMU, i.e. with the
> same headers.
> This was discussed earlier, for example in
> https://lists.nongnu.org/archive/html/qemu-devel/2021-06/msg02248.html
>
>>
>> I suspect you're trying to address the problem at the wrong level.
>
> What is the proper solution for the problem from your point of view?

I'll explain in more detail, but first I'd like you to answer my
question below.

>> Similar versioning issues exist with other helpers.  We've been doing
>> fine without QEMU providing unreliable advice on where they might sit in
>> the file system.  What makes this one different?
>
> This one is required to be *fully synchronized* with the existing build of 
> QEMU.
> Other helpers are probably less restrictive and do not have common
> structures definitions with the QEMU, otherwise they would face the
> same problem.
>
>>
>> >> What happens when you use the wrong helper?
>
> Our intention is that libvirt should never use the wrong RSS helper.
> But it does not have any ability to check which helper is compatible
> with the QEMU.
> QEMU can easily recognize the correct one.

You did not actually answer my question :)

So let's try again: if libvirt does use the wrong RSS helper, how does
the system behave?

>> >>
>> > UB - in most cases, eBPF program will work with wrong configurations.
>> > That's why the stamp was added.
>> >
>> > query-helper-paths checks the stamp only for RSS helper.
>>
>> I have no idea what you're talking about :)
>>
>> My best guess is that you're trying to tell me that attempting to work
>> with the wrong helper will fail cleanly due to some stamp check.  That
>> would be nice.
>>