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Message-ID: <fbeb5b70f4c6b036b71a58d4a2a13c534ed360a1.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 19:04:00 +0000
In-Reply-To: <35e45689-8225-7e5d-44ef-23479b563444@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>
         <13bc2ca60ca4e6d74c619e65502889961a08c3ff.camel@infradead.org>
         <35e45689-8225-7e5d-44ef-23479b563444@oracle.com>
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On Mon, 2020-11-30 at 18:41 +0000, Joao Martins wrote:
> int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> {
> ...
>         if (kvm_hv_hypercall_enabled(vcpu->kvm))
>                 return kvm_hv_hypercall(...);
>=20
>         if (kvm_xen_hypercall_enabled(vcpu->kvm))
>                 return kvm_xen_hypercall(...);
> ...
> }
>=20
> And on kvm_xen_hypercall() for the cases VMM offloads to demarshal what t=
he registers mean
> e.g. for event channel send 64-bit guest: RAX for opcode and RDI/RSI for =
cmd and port.

Right, although it's a little more abstract than that: "RDI/RSI for
arg#0, arg#1 respectively".

And those are RDI/RSI for 64-bit Xen, EBX/ECX for 32-bit Xen, and
RBX/RDI for Hyper-V. (And Hyper-V seems to use only the two, while Xen
theoretically has up to 6).

> The kernel logic wouldn't be much different at the core, so thought of ti=
hs consolidation.
> But the added complexity would have come from having to deal with two use=
rspace exit types
> -- indeed probably not worth the trouble as you pointed out.

Yeah, I think I'm just going to move the 'kvm_userspace_hypercall()'
from my patch to be 'kvm_xen_hypercall()' in a new xen.c but still
using KVM_EXIT_HYPERCALL. Then I can rebase your other patches on top
of that, with the evtchn bypass.


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