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

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 23, 2025, Manali Shukla wrote:
> On 4/16/2025 11:30 AM, Xiaoyao Li wrote:
> >>   +    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.
> 
> I didn't add this for development and debug purpose. I added this pr_info()
> to make it easy to find whether BUS Lock threshold is supported or not from
> dmesg.  I can remove it if you think it is not required.

Please remove it.  The user typically doesn't care.

> >> +        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.
> > 
> 
> Oh.., my bad, I will move the next patch before this one in v5.

No, do exactly as I suggested in v3.

 : I vote to split this into two patches: one to add the architectural collateral,
 : with the above as the changelog, and a second to actually implement support in
 : KVM.  Having the above background is useful, but it makes it quite hard to find
 : information on the KVM design and implementation.

I want this (and any other arch collateral I'm missing) in a separate patch so
that the background on what the hardware feature does is captured.  But I see no
reason to split KVM's implementation into multiple patches.

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index 2b59b9951c90..d1819c564b1c 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" }, \