linux-kernel - Re: [PATCH] x86/PCI: Use MMCONFIG by default for KVM guests

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Date:   Mon, 27 Jul 2020 17:04:39 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Julia Suvorova <jusual@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        "open list:VFIO DRIVER" <kvm@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] x86/PCI: Use MMCONFIG by default for KVM guests

On Wed, Jul 22, 2020 at 12:47 PM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> Julia Suvorova <jusual@...hat.com> writes:

> > Scanning for PCI devices at boot takes a long time for KVM guests. It
> > can be reduced if KVM will handle all configuration space accesses for
> > non-existent devices without going to userspace [1]. But for this to
> > work, all accesses must go through MMCONFIG.
> > This change allows to use pci_mmcfg as raw_pci_ops for 64-bit KVM
> > guests making MMCONFIG the default access method.

I'm not sure it won't break anything.

> > [1] https://lkml.org/lkml/2020/5/14/936

use Link: tag and better to use lore.kernel.org.

> This implies mmconfig access method is always functional (when present)
> for all KVM guests, regardless of hypervisor version/which KVM userspace
> is is use/... In case the assumption is true the patch looks good (to
> me) but in case it isn't or if we think that more control over this
> is needed we may want to introduce a PV feature bit for KVM.
>
> Also, I'm thinking about moving this to arch/x86/kernel/kvm.c: we can
> override x86_init.pci.arch_init and reassign raw_pci_ops after doing
> pci_arch_init().

% git grep -n -w x86_init.pci.arch_init -- arch/x86/
arch/x86/hyperv/hv_init.c:400:  x86_init.pci.arch_init = hv_pci_init;
arch/x86/kernel/apic/apic_numachip.c:203:       x86_init.pci.arch_init
= pci_numachip_init;
arch/x86/kernel/jailhouse.c:207:        x86_init.pci.arch_init
 = jailhouse_pci_arch_init;
arch/x86/pci/init.c:20: if (x86_init.pci.arch_init && !x86_init.pci.arch_init())
arch/x86/platform/intel-mid/intel-mid.c:172:    x86_init.pci.arch_init
= intel_mid_pci_init;
arch/x86/platform/olpc/olpc.c:309:              x86_init.pci.arch_init
= pci_olpc_init;
arch/x86/xen/enlighten_pv.c:1411:
x86_init.pci.arch_init = pci_xen_init;

Are you going to update all these? Or how this is supposed to work (I
may be missing something)?

-- 
With Best Regards,
Andy Shevchenko