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

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Date:   Mon, 7 Sep 2020 06:52:17 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        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
Subject: Re: [PATCH v2 0/3] KVM: x86: KVM_MEM_PCI_HOLE memory

On Fri, Sep 04, 2020 at 09:29:05AM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > Unless I'm mistaken, microvm doesn't even support PCI, does it?
> 
> Correct, no pci support right now.
> 
> We could probably wire up ecam (arm/virt style) for pcie support, once
> the acpi support for mictovm finally landed (we need acpi for that
> because otherwise the kernel wouldn't find the pcie bus).
> 
> Question is whenever there is a good reason to do so.  Why would someone
> prefer microvm with pcie support over q35?

The usual reasons to use pcie apply to microvm just the same.
E.g.: pass through of pcie devices?

> > If all of the above is true, this can be handled by adding "pci=lastbus=0"
> > as a guest kernel param to override its scanning of buses.  And couldn't
> > that be done by QEMU's microvm_fix_kernel_cmdline() to make it transparent
> > to the end user?
> 
> microvm_fix_kernel_cmdline() is a hack, not a solution.
> 
> Beside that I doubt this has much of an effect on microvm because
> it doesn't support pcie in the first place.
> 
> take care,
>   Gerd