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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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.

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().

Cc: kvm@vger.kernel.org

-- 
Vitaly