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

[Julia Suvorova <jusual@redhat.com>]

On Wed, Jul 22, 2020 at 11:43 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Julia Suvorova <jusual@redhat.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.
> >
> > [1] https://lkml.org/lkml/2020/5/14/936
> >
> > Signed-off-by: Julia Suvorova <jusual@redhat.com>
> > ---
> >  arch/x86/pci/direct.c      | 5 +++++
> >  arch/x86/pci/mmconfig_64.c | 3 +++
> >  2 files changed, 8 insertions(+)
> >
> > diff --git a/arch/x86/pci/direct.c b/arch/x86/pci/direct.c
> > index a51074c55982..8ff6b65d8f48 100644
> > --- a/arch/x86/pci/direct.c
> > +++ b/arch/x86/pci/direct.c
> > @@ -6,6 +6,7 @@
> >  #include <linux/pci.h>
> >  #include <linux/init.h>
> >  #include <linux/dmi.h>
> > +#include <linux/kvm_para.h>
> >  #include <asm/pci_x86.h>
> >
> >  /*
> > @@ -264,6 +265,10 @@ void __init pci_direct_init(int type)
> >  {
> >       if (type == 0)
> >               return;
> > +
> > +     if (raw_pci_ext_ops && kvm_para_available())
> > +             return;
> > +
> >       printk(KERN_INFO "PCI: Using configuration type %d for base access\n",
> >                type);
> >       if (type == 1) {
> > diff --git a/arch/x86/pci/mmconfig_64.c b/arch/x86/pci/mmconfig_64.c
> > index 0c7b6e66c644..9eb772821766 100644
> > --- a/arch/x86/pci/mmconfig_64.c
> > +++ b/arch/x86/pci/mmconfig_64.c
> > @@ -10,6 +10,7 @@
> >  #include <linux/init.h>
> >  #include <linux/acpi.h>
> >  #include <linux/bitmap.h>
> > +#include <linux/kvm_para.h>
> >  #include <linux/rcupdate.h>
> >  #include <asm/e820/api.h>
> >  #include <asm/pci_x86.h>
> > @@ -122,6 +123,8 @@ int __init pci_mmcfg_arch_init(void)
> >               }
> >
> >       raw_pci_ext_ops = &pci_mmcfg;
> > +     if (kvm_para_available())
> > +             raw_pci_ops = &pci_mmcfg;
> >
> >       return 1;
> >  }
>
> 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.

Ok, I'll introduce a feature bit and turn it on in QEMU.

> 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().

This might be a good idea.

Best regards, Julia Suvorova.