Using MMCONFIG instead of I/O ports cuts the number of config space accesses in half, which is faster on KVM and opens the door for additional optimizations such as Vitaly's "[PATCH 0/3] KVM: x86: KVM MEM_PCI_HOLE memory": https://lore.kernel.org/kvm/20200728143741.2718593-1-vkuznets@xxxxxxxxxx However, this change will not bring significant performance improvement unless it is running on x86 within a hypervisor. Moreover, allowing MMCONFIG access for addresses < 256 can be dangerous for some devices: see commit a0ca99096094 ("PCI x86: always use conf1 to access config space below 256 bytes"). That is why a special feature flag is needed. Introduce KVM_FEATURE_PCI_GO_MMCONFIG, which can be enabled when the configuration is known to be safe (e.g. in QEMU). Signed-off-by: Julia Suvorova <jusual@xxxxxxxxxx> --- Documentation/virt/kvm/cpuid.rst | 4 ++++ arch/x86/include/uapi/asm/kvm_para.h | 1 + arch/x86/kernel/kvm.c | 14 ++++++++++++++ 3 files changed, 19 insertions(+) diff --git a/Documentation/virt/kvm/cpuid.rst b/Documentation/virt/kvm/cpuid.rst index a7dff9186bed..711f2074877b 100644 --- a/Documentation/virt/kvm/cpuid.rst +++ b/Documentation/virt/kvm/cpuid.rst @@ -92,6 +92,10 @@ KVM_FEATURE_ASYNC_PF_INT 14 guest checks this feature bit async pf acknowledgment msr 0x4b564d07. +KVM_FEATURE_PCI_GO_MMCONFIG 15 guest checks this feature bit + before using MMCONFIG for all + PCI config accesses + KVM_FEATURE_CLOCSOURCE_STABLE_BIT 24 host will warn if no guest-side per-cpu warps are expeced in kvmclock diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h index 812e9b4c1114..5793f372cae0 100644 --- a/arch/x86/include/uapi/asm/kvm_para.h +++ b/arch/x86/include/uapi/asm/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/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c index df63786e7bfa..1ec73e6f25ce 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -33,6 +33,7 @@ #include <asm/hypervisor.h> #include <asm/tlb.h> #include <asm/cpuidle_haltpoll.h> +#include <asm/pci_x86.h> DEFINE_STATIC_KEY_FALSE(kvm_async_pf_enabled); @@ -715,6 +716,18 @@ static uint32_t __init kvm_detect(void) return kvm_cpuid_base(); } +static int __init kvm_pci_arch_init(void) +{ + if (raw_pci_ext_ops && + kvm_para_has_feature(KVM_FEATURE_PCI_GO_MMCONFIG)) { + pr_info("PCI: Using MMCONFIG for base access\n"); + raw_pci_ops = raw_pci_ext_ops; + return 0; + } + + return 1; +} + static void __init kvm_apic_init(void) { #if defined(CONFIG_SMP) @@ -726,6 +739,7 @@ static void __init kvm_apic_init(void) static void __init kvm_init_platform(void) { kvmclock_init(); + x86_init.pci.arch_init = kvm_pci_arch_init; x86_platform.apic_post_init = kvm_apic_init; } -- 2.25.4
