For some reason unknown to me, KVM allows writes to PMCR_EL0.N even
though the architecture specifies that field as RO. Make sure these
accesses conform to additional constraints imposed when the PMU is
partitioned.

Signed-off-by: Colton Lewis <coltonlewis@google.com>
---
 arch/arm64/kvm/pmu.c      | 2 +-
 arch/arm64/kvm/sys_regs.c | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/pmu.c b/arch/arm64/kvm/pmu.c
index 90fc088ce3d3..5f0847dc7d53 100644
--- a/arch/arm64/kvm/pmu.c
+++ b/arch/arm64/kvm/pmu.c
@@ -884,7 +884,7 @@ u64 kvm_pmu_accessible_counter_mask(struct kvm_vcpu *vcpu)
 u64 kvm_vcpu_read_pmcr(struct kvm_vcpu *vcpu)
 {
 	u64 pmcr = __vcpu_sys_reg(vcpu, PMCR_EL0);
-	u64 n = kvm_pmu_hpmn(vcpu);
+	u64 n = kvm_pmu_guest_num_counters(vcpu);
 
 	if (vcpu_has_nv(vcpu) && !vcpu_is_el2(vcpu))
 		n = FIELD_GET(MDCR_EL2_HPMN, __vcpu_sys_reg(vcpu, MDCR_EL2));
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 1ea7d092ec59..b64b60e297bd 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1266,7 +1266,9 @@ static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
 	 */
 	if (!kvm_vm_has_ran_once(kvm) &&
 	    !vcpu_has_nv(vcpu)	      &&
-	    new_n <= kvm_arm_pmu_get_max_counters(kvm))
+	    new_n <= kvm_arm_pmu_get_max_counters(kvm) &&
+	    (!kvm_vcpu_pmu_is_partitioned(vcpu) ||
+	     new_n <= kvm->arch.arm_pmu->hpmn_max))
 		kvm->arch.nr_pmu_counters = new_n;
 
 	mutex_unlock(&kvm->arch.config_lock);
-- 
2.50.0.714.g196bf9f422-goog

Hi Colton,

kernel test robot noticed the following build errors:

[auto build test ERROR on 79150772457f4d45e38b842d786240c36bb1f97f]

url:    https://github.com/intel-lab-lkp/linux/commits/Colton-Lewis/arm64-cpufeature-Add-cpucap-for-HPMN0/20250621-102220
base:   79150772457f4d45e38b842d786240c36bb1f97f
patch link:    https://lore.kernel.org/r/20250620221326.1261128-19-coltonlewis%40google.com
patch subject: [PATCH v2 17/23] KVM: arm64: Account for partitioning in PMCR_EL0 access
config: arm64-randconfig-r073-20250622 (https://download.01.org/0day-ci/archive/20250622/202506221711.tFNGpzj4-lkp@intel.com/config)
compiler: aarch64-linux-gcc (GCC) 8.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250622/202506221711.tFNGpzj4-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202506221711.tFNGpzj4-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from arch/arm64/include/asm/kvm_host.h:32,
                    from include/linux/kvm_host.h:45,
                    from arch/arm64/kvm/sys_regs.c:16:
   arch/arm64/include/asm/kvm_pmu.h:236:50: warning: 'struct arm_pmu' declared inside parameter list will not be visible outside of this definition or declaration
    static inline bool kvm_pmu_is_partitioned(struct arm_pmu *pmu)
                                                     ^~~~~~~
   arch/arm64/include/asm/kvm_pmu.h:241:52: warning: 'struct arm_pmu' declared inside parameter list will not be visible outside of this definition or declaration
    static inline u64 kvm_pmu_host_counter_mask(struct arm_pmu *pmu)
                                                       ^~~~~~~
   arch/arm64/include/asm/kvm_pmu.h:246:53: warning: 'struct arm_pmu' declared inside parameter list will not be visible outside of this definition or declaration
    static inline u64 kvm_pmu_guest_counter_mask(struct arm_pmu *pmu)
                                                        ^~~~~~~
   arch/arm64/kvm/sys_regs.c:856:6: warning: no previous prototype for 'pmu_access_el0_disabled' [-Wmissing-prototypes]
    bool pmu_access_el0_disabled(struct kvm_vcpu *vcpu)
         ^~~~~~~~~~~~~~~~~~~~~~~
   arch/arm64/kvm/sys_regs.c: In function 'access_pmu_evtyper':
   arch/arm64/kvm/sys_regs.c:1076:7: error: implicit declaration of function 'kvm_vcpu_pmu_is_partitioned'; did you mean 'kvm_pmu_is_partitioned'? [-Werror=implicit-function-declaration]
      if (kvm_vcpu_pmu_is_partitioned(vcpu))
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~
          kvm_pmu_is_partitioned
   arch/arm64/kvm/sys_regs.c: In function 'set_pmcr':
>> arch/arm64/kvm/sys_regs.c:1271:33: error: dereferencing pointer to incomplete type 'struct arm_pmu'
          new_n <= kvm->arch.arm_pmu->hpmn_max))
                                    ^~
   cc1: some warnings being treated as errors


vim +1271 arch/arm64/kvm/sys_regs.c

  1253	
  1254	static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
  1255			    u64 val)
  1256	{
  1257		u8 new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
  1258		struct kvm *kvm = vcpu->kvm;
  1259	
  1260		mutex_lock(&kvm->arch.config_lock);
  1261	
  1262		/*
  1263		 * The vCPU can't have more counters than the PMU hardware
  1264		 * implements. Ignore this error to maintain compatibility
  1265		 * with the existing KVM behavior.
  1266		 */
  1267		if (!kvm_vm_has_ran_once(kvm) &&
  1268		    !vcpu_has_nv(vcpu)	      &&
  1269		    new_n <= kvm_arm_pmu_get_max_counters(kvm) &&
  1270		    (!kvm_vcpu_pmu_is_partitioned(vcpu) ||
> 1271		     new_n <= kvm->arch.arm_pmu->hpmn_max))
  1272			kvm->arch.nr_pmu_counters = new_n;
  1273	
  1274		mutex_unlock(&kvm->arch.config_lock);
  1275	
  1276		/*
  1277		 * Ignore writes to RES0 bits, read only bits that are cleared on
  1278		 * vCPU reset, and writable bits that KVM doesn't support yet.
  1279		 * (i.e. only PMCR.N and bits [7:0] are mutable from userspace)
  1280		 * The LP bit is RES0 when FEAT_PMUv3p5 is not supported on the vCPU.
  1281		 * But, we leave the bit as it is here, as the vCPU's PMUver might
  1282		 * be changed later (NOTE: the bit will be cleared on first vCPU run
  1283		 * if necessary).
  1284		 */
  1285		val &= ARMV8_PMU_PMCR_MASK;
  1286	
  1287		/* The LC bit is RES1 when AArch32 is not supported */
  1288		if (!kvm_supports_32bit_el0())
  1289			val |= ARMV8_PMU_PMCR_LC;
  1290	
  1291		__vcpu_assign_sys_reg(vcpu, r->reg, val);
  1292		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
  1293	
  1294		return 0;
  1295	}
  1296	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

kernel test robot <lkp@intel.com> writes:

>     In file included from arch/arm64/include/asm/kvm_host.h:32,
>                      from include/linux/kvm_host.h:45,
>                      from arch/arm64/kvm/sys_regs.c:16:
>     arch/arm64/include/asm/kvm_pmu.h:236:50: warning: 'struct arm_pmu'  
> declared inside parameter list will not be visible outside of this  
> definition or declaration
>      static inline bool kvm_pmu_is_partitioned(struct arm_pmu *pmu)
>                                                       ^~~~~~~
>     arch/arm64/include/asm/kvm_pmu.h:241:52: warning: 'struct arm_pmu'  
> declared inside parameter list will not be visible outside of this  
> definition or declaration
>      static inline u64 kvm_pmu_host_counter_mask(struct arm_pmu *pmu)
>                                                         ^~~~~~~
>     arch/arm64/include/asm/kvm_pmu.h:246:53: warning: 'struct arm_pmu'  
> declared inside parameter list will not be visible outside of this  
> definition or declaration
>      static inline u64 kvm_pmu_guest_counter_mask(struct arm_pmu *pmu)
>                                                          ^~~~~~~
>     arch/arm64/kvm/sys_regs.c:856:6: warning: no previous prototype  
> for 'pmu_access_el0_disabled' [-Wmissing-prototypes]
>      bool pmu_access_el0_disabled(struct kvm_vcpu *vcpu)
>           ^~~~~~~~~~~~~~~~~~~~~~~
>     arch/arm64/kvm/sys_regs.c: In function 'access_pmu_evtyper':
>     arch/arm64/kvm/sys_regs.c:1076:7: error: implicit declaration of  
> function 'kvm_vcpu_pmu_is_partitioned'; did you  
> mean 'kvm_pmu_is_partitioned'? [-Werror=implicit-function-declaration]
>        if (kvm_vcpu_pmu_is_partitioned(vcpu))
>            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
>            kvm_pmu_is_partitioned
>     arch/arm64/kvm/sys_regs.c: In function 'set_pmcr':
>>> arch/arm64/kvm/sys_regs.c:1271:33: error: dereferencing pointer to  
>>> incomplete type 'struct arm_pmu'
>            new_n <= kvm->arch.arm_pmu->hpmn_max))
>                                      ^~
>     cc1: some warnings being treated as errors

Looks like the main problem here is struct arm_pmu is not defined if we
don't have CONFIG_ARM_PMU. I'll provide a dummy definition for that case
and swap out the field reference for a function call.

Secondary problem is making sure there is a prototype for
pmu_access_el0_disabled in that instance.

> vim +1271 arch/arm64/kvm/sys_regs.c

>    1253
>    1254	static int set_pmcr(struct kvm_vcpu *vcpu, const struct  
> sys_reg_desc *r,
>    1255			    u64 val)
>    1256	{
>    1257		u8 new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
>    1258		struct kvm *kvm = vcpu->kvm;
>    1259
>    1260		mutex_lock(&kvm->arch.config_lock);
>    1261
>    1262		/*
>    1263		 * The vCPU can't have more counters than the PMU hardware
>    1264		 * implements. Ignore this error to maintain compatibility
>    1265		 * with the existing KVM behavior.
>    1266		 */
>    1267		if (!kvm_vm_has_ran_once(kvm) &&
>    1268		    !vcpu_has_nv(vcpu)	      &&
>    1269		    new_n <= kvm_arm_pmu_get_max_counters(kvm) &&
>    1270		    (!kvm_vcpu_pmu_is_partitioned(vcpu) ||
>> 1271		     new_n <= kvm->arch.arm_pmu->hpmn_max))
>    1272			kvm->arch.nr_pmu_counters = new_n;
>    1273
>    1274		mutex_unlock(&kvm->arch.config_lock);
>    1275
>    1276		/*
>    1277		 * Ignore writes to RES0 bits, read only bits that are cleared on
>    1278		 * vCPU reset, and writable bits that KVM doesn't support yet.
>    1279		 * (i.e. only PMCR.N and bits [7:0] are mutable from userspace)
>    1280		 * The LP bit is RES0 when FEAT_PMUv3p5 is not supported on the  
> vCPU.
>    1281		 * But, we leave the bit as it is here, as the vCPU's PMUver might
>    1282		 * be changed later (NOTE: the bit will be cleared on first vCPU  
> run
>    1283		 * if necessary).
>    1284		 */
>    1285		val &= ARMV8_PMU_PMCR_MASK;
>    1286
>    1287		/* The LC bit is RES1 when AArch32 is not supported */
>    1288		if (!kvm_supports_32bit_el0())
>    1289			val |= ARMV8_PMU_PMCR_LC;
>    1290
>    1291		__vcpu_assign_sys_reg(vcpu, r->reg, val);
>    1292		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
>    1293
>    1294		return 0;
>    1295	}
>    1296

> --
> 0-DAY CI Kernel Test Service
> https://github.com/intel/lkp-tests/wiki