Re: [RFC PATCH v2 3/5] KVM: SVM: Enable Bus lock threshold exit

On 10/1/24 01:34, 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 (occurs after guest instruction finishes)

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.

When SVM_EXIT_BUS_LOCK occurs, the value of the current Bus Lock Threshold counter, which is '0', is loaded into the VMCB. This value must be reset to a default greater than '0' in bus_lock_exit handler in hypervisor, because the behavior of SVM_EXIT_BUS_LOCK is fault like, so the bus lock exit to userspace happens with the RIP pointing to the offending instruction (which generates buslocks).

So, if the value of the Bus Lock Threshold counter remains '0', the guest continuously exits with SVM_EXIT_BUS_LOCK.

The generated SVM_EXIT_BUS_LOCK in kvm will exit to user space by setting the KVM_RUN_BUS_LOCK flag in vcpu->run->flags, indicating to the user space that a bus lock has been detected in the guest.

Use the already available KVM capability KVM_CAP_X86_BUS_LOCK_EXIT to enable the feature. This feature is disabled by default, it can be enabled explicitly from user space.

More details about the Bus Lock Threshold feature can be found in AMD APM [1].

You should mention in the commit message that this implementation is set up to intercept every guest bus lock. The initial value of the threshold is 0 in the VMCB, so the very first bus lock will exit, set the threshold to 1 (so that the offending instruction can make forward progress) but then the value is at 0 again so the next bus lock will exit.

[1]: AMD64 Architecture Programmer's Manual Pub. 24593, April 2024, Vol 2, 15.14.5 Bus Lock Threshold. https://bugzilla.kernel.org/attachment.cgi?id=306250

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/nested.c | 12 ++++++++++++ arch/x86/kvm/svm/svm.c | 29 +++++++++++++++++++++++++++++ 4 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h index f0dea3750ca9..bad9723f40e1 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,

}; @@ -158,7 +159,9 @@ 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 1814b413fd57..abf6aed88cee 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_NPF 0x400 #define SVM_EXIT_AVIC_INCOMPLETE_IPI 0x401 #define SVM_EXIT_AVIC_UNACCELERATED_ACCESS 0x402 @@ -224,6 +225,7 @@ { 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_NPF, "npf" }, \ { SVM_EXIT_AVIC_INCOMPLETE_IPI, "avic_incomplete_ipi" }, \ { SVM_EXIT_AVIC_UNACCELERATED_ACCESS, "avic_unaccelerated_access" }, \

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c index 6f704c1037e5..670092d31f77 100644 --- a/arch/x86/kvm/svm/nested.c +++ b/arch/x86/kvm/svm/nested.c @@ -669,6 +669,12 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm, vmcb02->control.iopm_base_pa = vmcb01->control.iopm_base_pa; vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;

• /*

• * The bus lock threshold count should keep running across nested
  

• * transitions.
  

• * Copied the buslock threshold count from vmcb01 to vmcb02.
  

No need to start this part of the comment on a separate line. Also, s/Copied/Copy/ (ditto below).

• */
  

• vmcb02->control.bus_lock_counter = vmcb01->control.bus_lock_counter; /* Done at vmrun: asid. */

/* Also overwritten later if necessary. */ @@ -1035,6 +1041,12 @@ int nested_svm_vmexit(struct vcpu_svm *svm) }

• /*

• * The bus lock threshold count should keep running across nested
  

• * transitions.
  

• * Copied the buslock threshold count from vmcb02 to vmcb01.
  

• */
  

• vmcb01->control.bus_lock_counter = vmcb02->control.bus_lock_counter; nested_svm_copy_common_state(svm->nested.vmcb02.ptr, svm->vmcb01.ptr);

svm_switch_vmcb(svm, &svm->vmcb01); diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 5ab2c92c7331..41c773a40f99 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -1372,6 +1372,11 @@ 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);
  

• else

• svm_clr_intercept(svm, INTERCEPT_BUSLOCK);
  

Is the else path really needed?

Thanks, Tom

• if (sev_guest(vcpu->kvm)) sev_init_vmcb(svm);

@@ -3283,6 +3288,24 @@ static int invpcid_interception(struct kvm_vcpu *vcpu) return kvm_handle_invpcid(vcpu, type, gva); } +static int bus_lock_exit(struct kvm_vcpu *vcpu) +{

• struct vcpu_svm *svm = to_svm(vcpu);
• vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
• vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
• /*

• * Reload the counter with value greater than '0'.
  

• * The bus lock exit on SVM happens with RIP pointing to the guilty
  

• * instruction. So, reloading the value of bus_lock_counter to '0'
  

• * results into generating continuous bus lock exits.
  

• */
  

• svm->vmcb->control.bus_lock_counter = 1;
• return 0;

+}

static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = { [SVM_EXIT_READ_CR0] = cr_interception, [SVM_EXIT_READ_CR3] = cr_interception, @@ -3350,6 +3373,7 @@ static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = { [SVM_EXIT_CR4_WRITE_TRAP] = cr_trap, [SVM_EXIT_CR8_WRITE_TRAP] = cr_trap, [SVM_EXIT_INVPCID] = invpcid_interception,

• [SVM_EXIT_BUS_LOCK] = bus_lock_exit, [SVM_EXIT_NPF] = npf_interception, [SVM_EXIT_RSM] = rsm_interception, [SVM_EXIT_AVIC_INCOMPLETE_IPI] = avic_incomplete_ipi_interception,

@@ -5212,6 +5236,11 @@ static __init void svm_set_cpu_caps(void) kvm_cpu_cap_set(X86_FEATURE_SVME_ADDR_CHK); }

• if (cpu_feature_enabled(X86_VIRT_FEATURE_BUS_LOCK_THRESHOLD)) {

• pr_info("Bus Lock Threashold supported\n");
  

• kvm_caps.has_bus_lock_exit = true;
  

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