linux-kernel - Re: [PATCH v2 0/3] KVM: x86: KVM_MEM_PCI

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Message-ID: <20200921172114.GA24289@linux.intel.com>
Date:   Mon, 21 Sep 2020 10:21:14 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Julia Suvorova <jsuvorov@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        linux-kernel@...r.kernel.org, Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [PATCH v2 0/3] KVM: x86: KVM_MEM_PCI_HOLE memory

On Fri, Sep 18, 2020 at 08:34:37AM -0400, Michael S. Tsirkin wrote:
> On Fri, Sep 11, 2020 at 10:00:31AM -0700, Sean Christopherson wrote:
> > On Mon, Sep 07, 2020 at 07:32:23AM -0400, Michael S. Tsirkin wrote:
> > > I suspect microvms will need pci eventually. I would much rather KVM
> > > had an exit-less discovery mechanism in place by then because
> > > learning from history if it doesn't they will do some kind of
> > > hack on the kernel command line, and everyone will be stuck
> > > supporting that for years ...
> > 
> > Is it not an option for the VMM to "accurately" enumerate the number of buses?
> > E.g. if the VMM has devices on only bus 0, then enumerate that there is one
> > bus so that the guest doesn't try and probe devices that can't possibly exist.
> > Or is that completely non-sensical and/or violate PCIe spec?
> 
> 
> There is some tension here, in that one way to make guest boot faster
> is to defer hotplug of devices until after it booted.

Sorry, I didn't follow that, probably because my PCI knowledge is lacking.
What does device hotplug have to do with the number of buses enumerated to
the guest?