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

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Message-ID: <1ce5f5501214f1073f4eb86b2fdf3f54b2400057.camel@infradead.org>
Date:   Wed, 18 Jan 2023 14:26:49 +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 14:22 +0000, Andrew Cooper wrote:
> On 18/01/2023 2:06 pm, David Woodhouse wrote:
> > 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?
> 
> There is no actual thing called PVHVM.  That diagram has caused far more
> damage than good...

Perhaps so. Even CONFIG_XEN_PVHVM in the kernel is a nonsense, because
it's just automatically set based on (XEN && X86_LOCAL_APIC). And
CONFIG_XEN depends on X86_LOCAL_APIC anyway.

Which is why isn't never mattered that the vector callback handling was
under #ifdef CONFIG_XEN_PVHVM not just CONFIG_XEN.

> There's HVM (and by this, I mean the hypervisor's interpretation meaning
> VT-x or SVM), and a spectrum of things the guest kernel can do if it
> desires.
> 
> I'm pretty sure Linux knows all of them.

But don't we want to refrain from providing the legacy PC platform devices?


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