linux-kernel - Re: [PATCH] xen: Allow platform PCI interrupt to be shared

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Date:   Wed, 18 Jan 2023 14:06:40 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Durrant <paul@....org>
Subject: Re: [PATCH] xen: Allow platform PCI interrupt to be shared

On Wed, 2023-01-18 at 13:53 +0000, Andrew Cooper wrote:
> On 18/01/2023 12:22 pm, David Woodhouse wrote:
> > Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> > ---
> > What does xen_evtchn_do_upcall() exist for? Can we delete it? I don't
> > see it being called anywhere.
> 
> Seems the caller was dropped by
> cb09ea2924cbf1a42da59bd30a59cc1836240bcb, but the CONFIG_PVHVM looks
> bogus because the precondition to setting it up was being in a Xen HVM
> guest, and the guest is taking evtchns by vector either way.
> 
> PV guests use the entrypoint called exc_xen_hypervisor_callback which
> really ought to gain a PV in its name somewhere.  Also the comments look
> distinctly suspect.

Yeah. I couldn't *see* any asm or macro magic which would reference
xen_evtchn_do_upcall, and removing it from my build (with CONFIG_XEN_PV
enabled) also didn't break anything.

> Some tidying in this area would be valuable.

Indeed. I just need Paul or myself to throw in a basic XenStore
implementation so we can provide a PV disk, and I should be able to do
quickfire testing of PV guests too with 'qemu -kernel' and a PV shim.

PVHVM would be an entertaining thing to support too; I suppose that's
mostly a case of basing it on the microvm qemu platform, or perhaps
even *more* minimal x86-based platform?

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