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

[Julia Suvorova]

On Thu, Jul 23, 2020 at 1:19 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Wed, Jul 22, 2020 at 02:15:13AM +0200, Julia Suvorova wrote:
> > 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.
>
> The above *looks* like it's intended to be two paragraphs, which would
> be easier to read with a blank line between.
>
> The last sentence should say what the patch actually *does*, e.g.,
> "Use pci_mmcfg as raw_pci_ops ..."
>
> > [1] https://lkml.org/lkml/2020/5/14/936
>
> Please use a lore.kernel.org URL instead because it's more usable and
> I'd rather depend on kernel.org than lkml.org.
>
> > Signed-off-by: Julia Suvorova <jusual@xxxxxxxxxx>
> > ---
> >  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;
>
> The idea of using MMCONFIG for *all* config space, not just extended
> config space, makes sense to me, although the very long discussion at
> https://lore.kernel.org/lkml/20071225032605.29147200@xxxxxxxxxxxxxxxxxxxxx/
> makes me wary.  Of course I realize you're talking specifically about
> KVM, not doing this in general.
>
> But it doesn't seem right to make this specific to KVM, since it's not
> obvious to me that there's a basis in PCI for making this distinction.

Bugs that were fixed (or more accurately, avoided) by a0ca99096094
("PCI x86: always use conf1 to access config space below 256 bytes")
are still present. And to enable MMCONFIG for the entire config space,
we need to re-introduce all these fixes or at least identify affected
devices, which may be impossible.

We can avoid KVM-specific changes in the generic PCI code by
implementing x86_init.pci.arch_init inside KVM code, as Vitaly
suggested. What do you think?

Best regards, Julia Suvorova.