Re: [RFC PATCH v1] x86/split_lock: Provide KVM helper to log guest bus lock exits

[Manali Shukla <manali.shukla@amd.com>]

On 3/24/2026 11:34 PM, Borislav Petkov wrote:
> On Tue, Mar 24, 2026 at 12:40:02PM +0000, Manali Shukla wrote:
>> Bus locks monopolize the memory bus for thousands of cycles, stalling
>> all CPUs and degrading system performance for every workload on the
>> system. Cloud providers running multiple VMs need a way to identify
>> which guest is responsible, so the offending VM can be flagged.
>>
>> Bus lock VMEXITs on AMD are fault-style, i.e. the vCPU's RIP points to
>> the offending instruction at the time of the exit. Log guest's linear
>> RIP and vCPU info to give operators the data needed to attribute the bus
>> lock to a specific VM and instruction. Keep policy enforcement to
>> userspace via KVM_EXIT_X86_BUS_LOCK and let userspace implement
>> throttling or other mitigations.
>>
>> Reuse current->reported_split_lock to limit the warning to the once per
> 
> s/ to the once/ to once/
> 
>> vCPU thread. Note that, current->reported_split_lock tracks the vCPU
>> thread, not the individual guest process, i.e. the warning fires once
>> per vCPU for the lifetime of thread regardless of how many distinct
> 
> "... for the lifetime of the thread... "
> 
>> guest processes trigger bus locks on that vCPU.
> 
> I guess that's fine since you're ratelimited.
> 
>>
>> ----
> 
> What's that "----" supposed to denote here?
> 
>> The intent is purely observational — give hypervisor owners
>> visibility into which guest is generating bus locks so they can act
>> accordingly. No policy enforcement is done in the kernel. Suggestions
>> on the approach are welcome.
> 
> Oh, sounds like you want this paragraph to be...
> 
>>
>> Suggested-by: Nikunj A Dadhania <nikunj@amd.com>
>> Signed-off-by: Manali Shukla <manali.shukla@amd.com>
>> ---
> 
> <--- ... here, after the SOB block and above the diffstat.

Oh, my bad. will correct in v2.

> 
>>  arch/x86/include/asm/cpu.h     |  2 ++
>>  arch/x86/kernel/cpu/bus_lock.c | 19 +++++++++++++++++++
>>  arch/x86/kvm/svm/svm.c         |  1 +
>>  3 files changed, 22 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
>> index ad235dda1ded..4ac5011e10b2 100644
>> --- a/arch/x86/include/asm/cpu.h
>> +++ b/arch/x86/include/asm/cpu.h
>> @@ -29,6 +29,7 @@ unsigned int x86_stepping(unsigned int sig);
>>  extern void __init sld_setup(struct cpuinfo_x86 *c);
>>  extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
>>  extern bool handle_guest_split_lock(unsigned long ip);
>> +extern void handle_guest_bus_lock(unsigned long ip);
>>  extern void handle_bus_lock(struct pt_regs *regs);
>>  void split_lock_init(void);
>>  void bus_lock_init(void);
>> @@ -44,6 +45,7 @@ static inline bool handle_guest_split_lock(unsigned long ip)
>>  	return false;
>>  }
>>  
>> +static inline void handle_guest_bus_lock(unsigned long ip) {}
>>  static inline void handle_bus_lock(struct pt_regs *regs) {}
>>  static inline void split_lock_init(void) {}
>>  static inline void bus_lock_init(void) {}
>> diff --git a/arch/x86/kernel/cpu/bus_lock.c b/arch/x86/kernel/cpu/bus_lock.c
>> index fb166662bc0d..a6d231410535 100644
>> --- a/arch/x86/kernel/cpu/bus_lock.c
>> +++ b/arch/x86/kernel/cpu/bus_lock.c
>> @@ -292,6 +292,25 @@ bool handle_guest_split_lock(unsigned long ip)
>>  }
>>  EXPORT_SYMBOL_FOR_KVM(handle_guest_split_lock);
>>  
>> +void handle_guest_bus_lock(unsigned long ip)
> 
> Exported function better have a prefix:
> 
> bus_lock_guest_handle() or so.
> 
> But, the only reason this function is here is because you need sld_state.
> 
> So, instead of doing that, you can add a helper called
> 
> sld_enabled()
> 
> or so, which is exported and then you can put the guest-specific handling in
> svm.c in case you want to expand it in the future and there you use
> sld_enabled(), etc...
> 

Thank you for your suggestion, will fix the naming in v2.
sld_enabled() could be added in bus_lock.c and called from
bus_lock_exit() in svm.c. However, the other reason for keeping the
function in bus_lock.c is the current->reported_split_lock usage - it is
already used in split_lock_warn() and handle_guest_split_lock() in the
same file, so keeping all reported_split_lock accesses in bus_lock.c
seemed more maintainable. But happy to move it to svm.c, if you prefer
that approach.

>> +{
>> +	/*
>> +	 * Log bus lock exit when called from KVM exit handlers. Policy
>> +	 * enforcement is delegated to userspace via KVM_EXIT_X86_BUS_LOCK.
>> +	 * Let userspace implement throttling or other mitigations.
>> +	 *
>> +	 * Only log when split lock detection is active to respect system-wide
>> +	 * bus lock detection policy. Use the reported_split_lock flag to
>> +	 * prevent log spam from the same vCPU.
>> +	 */
>> +	if (sld_state != sld_off && !current->reported_split_lock) {
>> +		pr_warn_ratelimited("%s/%d buslock at rip: 0x%lx\n",
>> +				    current->comm, current->pid, ip);
>> +		current->reported_split_lock = 1;
>> +	}
>> +}
>> +EXPORT_SYMBOL_FOR_KVM(handle_guest_bus_lock);
> 
> Thx.
> 

-Manali