[RFC PATCH] target/i386: kvm: Enable KVM_FEATURE_PCI_GO_MMCONFIG CPUID bit

[Julia Suvorova]

This feature allows MMCONFIG to be used even to access the base PCI
config space [1]. This means increased performance: one access to
MMCONFIG instead of two conventional accesses to I/O ports.

Q35 makes no distinction in base or extended PCI config access to
MMCONFIG, MMCONFIG is always on, and in case it is is not initialized,
probing of PCI devices will fall back to normal process and use type1
access.

Enable the feature unconditionally.

[1]: https://lore.kernel.org/kvm/20200730193510.578309-1-jusual@redhat.com/

Signed-off-by: Julia Suvorova <jusual@redhat.com>
---
The feature is in the review phase.

 include/standard-headers/asm-x86/kvm_para.h | 1 +
 target/i386/cpu.c                           | 3 ++-
 target/i386/kvm.c                           | 1 +
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/standard-headers/asm-x86/kvm_para.h 
b/include/standard-headers/asm-x86/kvm_para.h
index 07877d3295..52eeba9067 100644
--- a/include/standard-headers/asm-x86/kvm_para.h
+++ b/include/standard-headers/asm-x86/kvm_para.h
@@ -32,6 +32,7 @@
 #define KVM_FEATURE_POLL_CONTROL       12
 #define KVM_FEATURE_PV_SCHED_YIELD     13
 #define KVM_FEATURE_ASYNC_PF_INT       14
+#define KVM_FEATURE_PCI_GO_MMCONFIG    15
 
 #define KVM_HINTS_REALTIME      0
 
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 588f32e136..5509523bb3 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -810,7 +810,7 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
             "kvmclock", "kvm-nopiodelay", "kvm-mmu", "kvmclock",
             "kvm-asyncpf", "kvm-steal-time", "kvm-pv-eoi", "kvm-pv-unhalt",
             NULL, "kvm-pv-tlb-flush", NULL, "kvm-pv-ipi",
-            "kvm-poll-control", "kvm-pv-sched-yield", NULL, NULL,
+            "kvm-poll-control", "kvm-pv-sched-yield", NULL, 
"kvm-pci-go-mmconfig",
             NULL, NULL, NULL, NULL,
             NULL, NULL, NULL, NULL,
             "kvmclock-stable-bit", NULL, NULL, NULL,
@@ -4141,6 +4141,7 @@ static PropValue kvm_default_props[] = {
     { "acpi", "off" },
     { "monitor", "off" },
     { "svm", "off" },
+    { "kvm-pci-go-mmconfig", "on" },
     { NULL, NULL },
 };
 
diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index 6f18d940a5..0069e945e6 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -440,6 +440,7 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t 
function,
         if (!kvm_irqchip_in_kernel()) {
             ret &= ~(1U << KVM_FEATURE_PV_UNHALT);
         }
+        ret |= 1U << KVM_FEATURE_PCI_GO_MMCONFIG;
     } else if (function == KVM_CPUID_FEATURES && reg == R_EDX) {
         ret |= 1U << KVM_HINTS_REALTIME;
         found = 1;
-- 
2.25.4