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

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Message-Id: <20200722001513.298315-1-jusual@redhat.com>
Date:   Wed, 22 Jul 2020 02:15:13 +0200
From:   Julia Suvorova <jusual@...hat.com>
To:     linux-pci@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Julia Suvorova <jusual@...hat.com>
Subject: [PATCH] x86/PCI: Use MMCONFIG by default for KVM guests

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@...hat.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;
 }
-- 
2.25.4