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Message-ID: <13bc2ca60ca4e6d74c619e65502889961a08c3ff.camel@infradead.org>
Subject: Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd
From:   David Woodhouse <dwmw2@infradead.org>
To:     Joao Martins <joao.m.martins@oracle.com>,
        Ankur Arora <ankur.a.arora@oracle.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Mon, 30 Nov 2020 18:01:25 +0000
In-Reply-To: <05661003-64f0-a32a-5659-6463d4806ef9@oracle.com>
References: <20190220201609.28290-1-joao.m.martins@oracle.com>
         <20190220201609.28290-12-joao.m.martins@oracle.com>
         <874d1fa922cb56238676b90bbeeba930d0706500.camel@infradead.org>
         <e83f6438-7256-1dc8-3b13-5498fd5bbed1@oracle.com>
         <18e854e2a84750c2de2d32384710132b83d84286.camel@infradead.org>
         <0b9d3901-c10b-effd-6278-6afd1e95b09e@oracle.com>
         <315ea414c2bf938978f7f2c0598e80fa05b4c07b.camel@infradead.org>
         <05661003-64f0-a32a-5659-6463d4806ef9@oracle.com>
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On Mon, 2020-11-30 at 17:15 +0000, Joao Martins wrote:
> On 11/30/20 4:48 PM, David Woodhouse wrote:
> > On Mon, 2020-11-30 at 15:08 +0000, Joao Martins wrote:
> > > On 11/30/20 12:55 PM, David Woodhouse wrote:
> > > > On Mon, 2020-11-30 at 12:17 +0000, Joao Martins wrote:
> > > > > On 11/30/20 9:41 AM, David Woodhouse wrote:
> > > > > > On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote:
> > > > >=20
> > > > > One thing I didn't quite do at the time, is the whitelisting of u=
nregistered
> > > > > ports to userspace.

...

> But felt it was still worth having this discussion ... should this be
> considered or discarded. I suppose we stick with the later for now.

Ack. Duly discarded :)

> > > > > Perhaps eventfd could be a way to express this? Like if you regis=
ter
> > > > > without an eventfd it's offloaded, otherwise it's assigned to use=
rspace,
> > > > > or if neither it's then returned an error without bothering the V=
MM.
> > > >=20
> > > > I much prefer the simple model where the *only* event channels that=
 the
> > > > kernel knows about are the ones it's expected to handle.
> > > >=20
> > > > For any others, the bypass doesn't kick in, and userspace gets the
> > > > KVM_EXIT_HYPERCALL exit.
> > > >=20
> > >=20
> > > /me nods
> > >=20
> > > I should comment on your other patch but: if we're going to make it g=
eneric for
> > > the userspace hypercall handling, might as well move hyper-v there to=
o. In this series,
> > > I added KVM_EXIT_XEN, much like it exists KVM_EXIT_HYPERV -- but with=
 a generic version
> > > I wonder if a capability could gate KVM_EXIT_HYPERCALL to handle both=
 guest types, while
> > > disabling KVM_EXIT_HYPERV. But this is probably subject of its own se=
parate patch :)
> >=20
> > There's a limit to how much consolidation we can do because the ABI is
> > different; the args are in different registers.
> >=20
>=20
> Yes. It would be optionally enabled of course and VMM would have to adjus=
t to the new ABI
> -- surely wouldn't want to break current users of KVM_EXIT_HYPERV.

True, but that means we'd have to keep KVM_EXIT_HYPERV around anyway,
and can't actually *remove* it. The "consolidation" gives us more
complexity, not less.

> > I do suspect Hyper-V should have marshalled its arguments into the
> > existing kvm_run->arch.hypercall and used KVM_EXIT_HYPERCALL but I
> > don't think it makes sense to change it now since it's a user-facing
> > ABI. I don't want to follow its lead by inventing *another* gratuitous
> > exit type for Xen though.
> >=20
>=20
> I definitely like the KVM_EXIT_HYPERCALL better than a KVM_EXIT_XEN users=
pace
> exit type ;)
>=20
> But I guess you still need to co-relate a type of hypercall (Xen guest ca=
p enabled?) to
> tell it's Xen or KVM to specially enlighten certain opcodes (EVTCHNOP_sen=
d).

Sure, but if the VMM doesn't know what kind of guest it's hosting, we
have bigger problems... :)


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