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KVM: SVM: Add architectural definitions/assets for Bus Lock Threshold

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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>
Link: https://lore.kernel.org/r/20250502050346.14274-4-manali.shukla@amd.com
[sean: rewrite shortlog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Nikunj A Dadhania 2025-05-02 05:03:44 +00:00 • committed by Sean Christopherson
parent faad6645e1
commit 827547bc3a
2 changed files with 7 additions and 2 deletions
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7
arch/x86/include/asm/svm.h
View file

@ -116,6 +116,7 @@ enum {
INTERCEPT_INVPCID,
INTERCEPT_MCOMMIT,
INTERCEPT_TLBSYNC,
INTERCEPT_BUSLOCK,
INTERCEPT_IDLE_HLT = 166,
};
@ -159,10 +160,12 @@ 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[40];
u8 reserved_8[16];
u16 bus_lock_counter; /* Offset 0x120 */
u8 reserved_9[22];
u64 allowed_sev_features; /* Offset 0x138 */
u64 guest_sev_features; /* Offset 0x140 */
u8 reserved_9[664];
u8 reserved_10[664];
/*
* Offset 0x3e0, 32 bytes reserved
* for use by hypervisor/software.

2
arch/x86/include/uapi/asm/svm.h
View file

@ -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" }, \