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

[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

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

[Eduardo Habkost]

On Fri, Jul 31, 2020 at 08:49:38PM +0200, Julia Suvorova wrote:
> 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" },

You'll need a TYPE_X86_CPU.kvm-pci-go-mmconfig=off entry in
pc_compat_5_1 to keep guest ABI compatibility on older machine
types.

pc_compat_5_1 is introduced by
https://lore.kernel.org/qemu-devel/20200728094645.272149-1-cohuck@redhat.com/
("hw: add compat machines for 5.2").

>      { 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;

On most cases, enabling a feature unconditionally on
kvm_arch_get_supported_cpuid() would be a mistake, but this flag
seems to be an exception to the rule.

A comment here explaining why it is really safe to enable it
unconditionally would be welcome.

>      } else if (function == KVM_CPUID_FEATURES && reg == R_EDX) {
>          ret |= 1U << KVM_HINTS_REALTIME;
>          found = 1;
> -- 
> 2.25.4
> 

-- 
Eduardo