Re: [PATCH 09/14] KVM: PPC: Add generic single register ioctls

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Alexander Graf <agraf@suse.de>
Cc: kvm-ppc@vger.kernel.org, kvm list <kvm@vger.kernel.org>
Subject: Re: [PATCH 09/14] KVM: PPC: Add generic single register ioctls
Date: Wed, 23 Nov 2011 10:47:08 -0200	[thread overview]
Message-ID: <20111123124708.GA13448@amt.cnet> (raw)
In-Reply-To: <35750AAB-B919-4F7B-B60E-C4ADE9124048@suse.de>

On Wed, Nov 16, 2011 at 12:45:45AM +0100, Alexander Graf wrote:
> 
> On 10.11.2011, at 18:35, Marcelo Tosatti wrote:
> 
> > On Thu, Nov 10, 2011 at 05:49:42PM +0100, Alexander Graf wrote:
> >>>> Documentation/virtual/kvm/api.txt |   47 ++++++++++++++++++++++++++++++++++
> >>>> arch/powerpc/kvm/powerpc.c        |   51 +++++++++++++++++++++++++++++++++++++
> >>>> include/linux/kvm.h               |   32 +++++++++++++++++++++++
> >>>> 3 files changed, 130 insertions(+), 0 deletions(-)
> >>> I don't see the benefit of this generalization, the current structure where
> >>> context information is hardcoded in the data transmitted works well.
> >> 
> >> Well, unfortunately it doesn't work quite as well for us because we
> >> are a much more evolving platform. Also, there are a lot of edges
> >> and corners of the architecture that simply aren't implemented in
> >> KVM as of now. I want to have something extensible enough so we
> >> don't break the ABI along the way.
> > 
> > You still have to agree on format between userspace and kernel, right?
> > If either party fails to conform to that, you're doomed.
> 
> Yes, but we can shove registers back and forth without allocating 8kb of ram each time. If all we need to do is poke one register, we poke one register. If we poke 10, we poke the 10 we need to touch.
> 
> > The problem with two interfaces is potential ambiguity: is
> > register X implemented through KVM_GET_ONE_REG and also through
> > KVM_GET_XYZ_REGISTER_SET ? If its accessible by two interfaces, what is
> > the register writeback order? Is there a plan to convert, etc.
> 
> Why writeback order? Register modification operations should always happen from the same thread the vCPU would run on at the end of the day, no?

Yes, but there is a specified order which the registers must be written
back, in case there are dependencies between them (the QEMU x86's code
does its best to document these dependencies).

All i'm saying is that two distinct interfaces make it potentially
confusing for the programmer. That said, its up to Avi to decide.

> Avi wanted to go as far as making this a syscall interface even.

Ok, then full convertion work must be done.

> > If you agree these concerns are valid, perhaps this interface can be PPC
> > specific.
> 
> I can always make it PPC specific, but I believe it would make sense as a generic interface for everyone, similar to how ENABLE_CAP can make sense for any arch.
> 
> 
> Alex