Re: [PATCH v4 3/5] KVM: SVM: Enable Bus lock threshold exit

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/24/2025 9:02 PM, Manali Shukla wrote:
> From: Nikunj A Dadhania <nikunj@amd.com>
> 
> Virtual machines can exploit bus locks to degrade the performance of
> the system. Bus locks can be caused by Non-WB(Write back) and
> misaligned locked RMW (Read-modify-Write) instructions and require
> systemwide synchronization among all processors which can result into
> significant performance penalties.
> 
> To address this issue, the Bus Lock Threshold feature is introduced to
> provide ability to hypervisor to restrict guests' capability of
> initiating mulitple buslocks, thereby preventing system slowdowns.
> 
> Support for the buslock threshold is indicated via CPUID function
> 0x8000000A_EDX[29].
> 
> On the processors that support the Bus Lock Threshold feature, the
> VMCB provides a Bus Lock Threshold enable bit and an unsigned 16-bit
> Bus Lock threshold count.
> 
> VMCB intercept bit
> VMCB Offset     Bits    Function
> 14h             5       Intercept bus lock operations
> 
> Bus lock threshold count
> VMCB Offset     Bits    Function
> 120h            15:0    Bus lock counter
> 
> When a VMRUN instruction is executed, the bus lock threshold count is
> loaded into an internal count register. Before the processor executes
> a bus lock in the guest, it checks the value of this register:
> 
>   - If the value is greater than '0', the processor successfully
>     executes the bus lock and decrements the count.
> 
>   - If the value is '0', the bus lock is not executed, and a #VMEXIT to
>     the VMM is taken.
> 
> The bus lock threshold #VMEXIT is reported to the VMM with the VMEXIT
> code A5h, SVM_EXIT_BUS_LOCK.
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> Co-developed-by: Manali Shukla <manali.shukla@amd.com>
> Signed-off-by: Manali Shukla <manali.shukla@amd.com>
> ---
>   arch/x86/include/asm/svm.h      | 5 ++++-
>   arch/x86/include/uapi/asm/svm.h | 2 ++
>   arch/x86/kvm/svm/svm.c          | 8 ++++++++
>   3 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> index 9b7fa99ae951..9dc54da1835a 100644
> --- a/arch/x86/include/asm/svm.h
> +++ b/arch/x86/include/asm/svm.h
> @@ -116,6 +116,7 @@ enum {
>   	INTERCEPT_INVPCID,
>   	INTERCEPT_MCOMMIT,
>   	INTERCEPT_TLBSYNC,
> +	INTERCEPT_BUSLOCK,
>   	INTERCEPT_IDLE_HLT = 166,
>   };
>   
> @@ -159,7 +160,9 @@ struct __attribute__ ((__packed__)) vmcb_control_area {
>   	u64 avic_physical_id;	/* Offset 0xf8 */
>   	u8 reserved_7[8];
>   	u64 vmsa_pa;		/* Used for an SEV-ES guest */
> -	u8 reserved_8[720];
> +	u8 reserved_8[16];
> +	u16 bus_lock_counter;	/* Offset 0x120 */
> +	u8 reserved_9[702];
>   	/*
>   	 * Offset 0x3e0, 32 bytes reserved
>   	 * for use by hypervisor/software.
> diff --git a/arch/x86/include/uapi/asm/svm.h b/arch/x86/include/uapi/asm/svm.h
> index ec1321248dac..9c640a521a67 100644
> --- a/arch/x86/include/uapi/asm/svm.h
> +++ b/arch/x86/include/uapi/asm/svm.h
> @@ -95,6 +95,7 @@
>   #define SVM_EXIT_CR14_WRITE_TRAP		0x09e
>   #define SVM_EXIT_CR15_WRITE_TRAP		0x09f
>   #define SVM_EXIT_INVPCID       0x0a2
> +#define SVM_EXIT_BUS_LOCK			0x0a5
>   #define SVM_EXIT_IDLE_HLT      0x0a6
>   #define SVM_EXIT_NPF           0x400
>   #define SVM_EXIT_AVIC_INCOMPLETE_IPI		0x401
> @@ -225,6 +226,7 @@
>   	{ SVM_EXIT_CR4_WRITE_TRAP,	"write_cr4_trap" }, \
>   	{ SVM_EXIT_CR8_WRITE_TRAP,	"write_cr8_trap" }, \
>   	{ SVM_EXIT_INVPCID,     "invpcid" }, \
> +	{ SVM_EXIT_BUS_LOCK,     "buslock" }, \
>   	{ SVM_EXIT_IDLE_HLT,     "idle-halt" }, \
>   	{ SVM_EXIT_NPF,         "npf" }, \
>   	{ SVM_EXIT_AVIC_INCOMPLETE_IPI,		"avic_incomplete_ipi" }, \
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 8abeab91d329..244e099e7262 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1375,6 +1375,9 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>   		svm->vmcb->control.int_ctl |= V_GIF_ENABLE_MASK;
>   	}
>   
> +	if (vcpu->kvm->arch.bus_lock_detection_enabled)
> +		svm_set_intercept(svm, INTERCEPT_BUSLOCK);
> +
>   	if (sev_guest(vcpu->kvm))
>   		sev_init_vmcb(svm);
>   
> @@ -5299,6 +5302,11 @@ static __init void svm_set_cpu_caps(void)
>   		kvm_cpu_cap_set(X86_FEATURE_SVME_ADDR_CHK);
>   	}
>   
> +	if (cpu_feature_enabled(X86_FEATURE_BUS_LOCK_THRESHOLD)) {
> +		pr_info("Bus Lock Threshold supported\n");

It will be printed every time kvm-amd.ko module gets loaded.

I think it's for your development and debug purpose. Comparing to the 
existing features in svm_set_cpu_caps(), nothing makes it special for 
BUS_LOCK_THRESHOLD to require a kernel message. So I think we can just 
remove it.

> +		kvm_caps.has_bus_lock_exit = true;

Besides, this patch doesn't ensure the bisectability. It allows 
userspace to enable KVM_BUS_LOCK_DETECTION_EXIT and set intercept of 
INTERCEPT_BUSLOCK but without providing the handler.

So either move next patch before it or just merge them.

> +	}
> +
>   	/* CPUID 0x80000008 */
>   	if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) ||
>   	    boot_cpu_has(X86_FEATURE_AMD_SSBD))