linux-kernel - Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd

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Date:   Mon, 30 Nov 2020 19:04:00 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Joao Martins <joao.m.martins@...cle.com>,
        Ankur Arora <ankur.a.arora@...cle.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd

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(...);
> 
>         if (kvm_xen_hypercall_enabled(vcpu->kvm))
>                 return kvm_xen_hypercall(...);
> ...
> }
> 
> And on kvm_xen_hypercall() for the cases VMM offloads to demarshal what the 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 tihs consolidation.
> But the added complexity would have come from having to deal with two userspace 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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