Re: [PATCH v4 5/8] arm/vm_event: get/set registers

From: Razvan Cojocaru <rcojocaru@bitdefender.com>
To: Jan Beulich <JBeulich@suse.com>, Tamas K Lengyel <tamas@tklengyel.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
	Julien Grall <julien.grall@arm.com>,
	Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: [PATCH v4 5/8] arm/vm_event: get/set registers
Date: Thu, 2 Jun 2016 11:26:34 +0300	[thread overview]
Message-ID: <8da9b1a2-450a-62e5-2f0c-89f58a7857be@bitdefender.com> (raw)
In-Reply-To: <574FFDDA02000078000F0B92@prv-mh.provo.novell.com>

On 06/02/2016 10:35 AM, Jan Beulich wrote:
> The criteria for inclusion or exclusion should
> follow a predictable model. I.e. if someone comes along and says
> "I need register Y", then there should be rules that (s)he can apply
> up front to determine what (at least in the vast majority of cases)
> the response is going to be. You saying "I need only this arbitrary
> subset of registers" is completely in-transparent, as it leaves open
> why this is so, and why this would also be so for others. I.e. you'd
> at least need to answer the question why (as an example) you
> need x5 included, but not x25, despite both registers being equal
> from an architecture pov. An answer like "My application gets away
> with it" is not acceptable here.

There will be trade-offs. Again, my preference would be to have a
multi-page ring buffer with all the VCPU context included in the
request, always. This would work best as far as both completeness and
performance goes.

But that is obviously not a trivial change and I can see how Tamas would
prefer to get things done sooner along the lines of the pre-existing
design (whatever its merits and demerits are).

As long as we're using a smaller ring buffer, the answer (at least as
I've meant it) is not "_my_ application gets away with it" - which is
indeed not the most reasonable statement, but "_current_ introspection
applications - of which my application is one of - don't need more than
this at this time". So in the spirit of "the rest of the context is a
hypercall away" + we don't have that much space available in the ring
buffer, the whole context is not included. The assumption here being
that once another application (or even our application, at a later time)
needs to enlarge the set of registers sent with a request, it will be
requested here and we can discuss what that entails.

Thanks,
Razvan

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