KVM: RISC-V: SBI FWFT: Fix stale feature exposure after runtime extension changes

Fix a bug where FWFT features could be incorrectly exposed to guests
after userspace disables their dependent ISA extensions at runtime.

The 'supported' field in kvm_sbi_fwft_config was set once during vCPU
initialization based on the initial hardware/extension availability.
However, when userspace subsequently disables ISA extensions via the KVM
ONE_REG interface, the 'supported' field was not updated. This caused
the following issues:
1. FWFT features would remain visible and accessible to guests even
   after their prerequisite ISA extensions were disabled
2. Guests could configure FWFT features that depend on disabled
   extensions, leading to undefined behavior
3. The static 'supported' flag and the dynamic supported() callback
   could disagree about feature availability

The fix introduces a two-layer checking mechanism:
1. Add an optional init() callback to the kvm_sbi_fwft_feature structure
   for features that require hardware probing during initialization. This
   separates the one-time hardware detection logic from the runtime
   availability check.
2. Add runtime checks in all FWFT-related functions that call
   feature->supported(vcpu) if the callback exists. This ensures feature
   availability is re-evaluated based on the current ISA extension state.

This approach maintains the cached 'supported' field for initialization-
time decisions while ensuring runtime availability is always determined
by the current vCPU configuration, not initialization-time snapshots.

Fixes: 6b72fd170592 ("RISC-V: KVM: add support for FWFT SBI extension")
Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260601-kvm-get_reg_list-v2-v5-3-415d08a2813b@sifive.com
Signed-off-by: Anup Patel <anup@brainfault.org>

diff --git a/arch/riscv/kvm/vcpu_sbi_fwft.c b/arch/riscv/kvm/vcpu_sbi_fwft.c
index aee951f2b8e63deffe3b44ea9b9327c813234f19..ab39ac464ffd817035e4938131562bc80a4ecf88 100644 (file)
--- a/arch/riscv/kvm/vcpu_sbi_fwft.c
+++ b/arch/riscv/kvm/vcpu_sbi_fwft.c
@@ -303,6 +303,8 @@ static int kvm_fwft_get_feature(struct kvm_vcpu *vcpu, u32 feature,
 
        if (!tconf->supported || !tconf->enabled)
                return SBI_ERR_NOT_SUPPORTED;
+       else if (tconf->feature->supported && !tconf->feature->supported(vcpu))
+               return SBI_ERR_NOT_SUPPORTED;
 
        *conf = tconf;
 
@@ -433,6 +435,8 @@ static unsigned long kvm_sbi_ext_fwft_get_reg_count(struct kvm_vcpu *vcpu)
                conf = kvm_sbi_fwft_get_config(vcpu, feature->id);
                if (!conf || !conf->supported)
                        continue;
+               else if (conf->feature->supported && !conf->feature->supported(vcpu))
+                       continue;
 
                ret++;
        }
@@ -455,6 +459,8 @@ static int kvm_sbi_ext_fwft_get_reg_id(struct kvm_vcpu *vcpu, int index, u64 *re
                conf = kvm_sbi_fwft_get_config(vcpu, feature->id);
                if (!conf || !conf->supported)
                        continue;
+               else if (conf->feature->supported && !conf->feature->supported(vcpu))
+                       continue;
 
                if (index == idx) {
                        *reg_id = KVM_REG_RISCV |
@@ -490,6 +496,8 @@ static int kvm_sbi_ext_fwft_get_reg(struct kvm_vcpu *vcpu, unsigned long reg_num
        conf = kvm_sbi_fwft_get_config(vcpu, feature->id);
        if (!conf || !conf->supported)
                return -ENOENT;
+       else if (conf->feature->supported && !conf->feature->supported(vcpu))
+               return -ENOENT;
 
        switch (reg_num - feature->first_reg_num) {
        case 0:
@@ -527,6 +535,8 @@ static int kvm_sbi_ext_fwft_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num
        conf = kvm_sbi_fwft_get_config(vcpu, feature->id);
        if (!conf || !conf->supported)
                return -ENOENT;
+       else if (conf->feature->supported && !conf->feature->supported(vcpu))
+               return -ENOENT;
 
        switch (reg_num - feature->first_reg_num) {
        case 0: