Re: [Qemu-devel] Question about add AF_ALG backend for virtio-crypto

[Gonglei (Arei)]

>
> > >
> > > >
> > > > >
> > > > > On Mon, Jan 09, 2017 at 01:43:10PM +0000, Stefan Hajnoczi wrote:
> > > > > > On Mon, Jan 09, 2017 at 03:04:55PM +0800, Longpeng (Mike) wrote:
> > > > > > > I'm one of Gonglei's virtio-crypto project members, and we plan to
> add a
> > > > > AF_ALG
> > > > > > > backend for virtio-crypto(there's only builtin-backend currently).
> > > > > > >
> > > > > > > I found that Catalin, Paolo and Stefan had discussed about this in
> 2015
> > > > > > > (http://www.spinics.net/lists/kvm/msg115457.html), but it seems
> that
> > > > > Catalin
> > > > > > > didn't do it, so I'm confuse about wether it is need to add a 
> > > > > > > AF_ALG
> > > > > backend.
> > > > > > >
> > > > > > > Do you have any suggestion? Thanks :)
> > > > > >
> > > > > > I have no objections to an AF_ALG backend in QEMU.
> > > > >
> > > > > Rather than do another backend for virtio-crypto, IMHO, we should have
> > > > > an AF_ALG impl of the crypto/ APIs. That way any potential performance
> > > > > benefits will enhance our LUKS encryption code too.
> > > > >
> > > > According to the currently schemas of crypto/ APIs, we can't choose the
> > > > specific backend dynamically. This is a limitation for virtio-crypto
> > > > device I think.
> > >
> > > Do we really need to be able to choose the backend explicitly. If the 
> > > AF_ALG
> > > backend is faster, why would you simply not use that automatically if it 
> > > is
> > > available.
> >
> > Can we realize the purpose based on the crypto/ APIs? IIUC the crypto
> > subsystem chooses a backend during the building according to the specific
> priority,
> > nettle > gcrypt > cipher-builtin.
> >
> > If we add an AF_ALG implementation for crypto subsystem, shall we set it
> > as the highest priority? If so, other backends won't be used since AF_ALG
> > is always available. If not, how can we use AF_ALG backend for crypto/ API?
> >
> > Please correct me if I'm missing something.
> 
> While AF_ALG has been available for a while, not all features have. For
> example AEAD support was only added in kernel 4.1
> 
OK.

> So if we had AF_ALG in QEMU, we would have to have a stacked impl, where
> we try AF_ALG and then fallback to the current code when QEMU runs on a
> kernel lacking the feature needed.
> 
It makes sense though the implementation will be more complicate
since the code should identify the different error codes are returned
by AF_ALG APIs.

> We could potentially also have a global arg to switch backends e.g.
> 
>  -crypto-backend [afalg|builtin]
> 
So the backend here is not the cryptodev backend?

Thanks,
-Gonglei