[PATCH v5 2/5] KVM: RISC-V: SBI FWFT: Add optional init() callback for hardware probing

[Yong-Xuan Wang]

Add an optional init() callback to separate one-time hardware probing
from runtime availability checks. For pointer masking, this allows
probing supported PMM lengths during initialization while checking ISA
extension availability at runtime.

Fix try_to_set_pmm() to restore the previous HENVCFG.PMM value after
probing, preventing side effects from hardware detection. Add preemption
protection to ensure CSR probe sequences complete atomically on the same
CPU.

Fixes: 6f576fc0aeb9 ("RISC-V: KVM: Add support for SBI_FWFT_POINTER_MASKING_PMLEN")

Signed-off-by: Yong-Xuan Wang <yongxuan.wang at sifive.com>
---
 arch/riscv/kvm/vcpu_sbi_fwft.c | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/kvm/vcpu_sbi_fwft.c b/arch/riscv/kvm/vcpu_sbi_fwft.c
index 5e4aafb0cbf1..aee951f2b8e6 100644
--- a/arch/riscv/kvm/vcpu_sbi_fwft.c
+++ b/arch/riscv/kvm/vcpu_sbi_fwft.c
@@ -35,6 +35,16 @@ struct kvm_sbi_fwft_feature {
 	 */
 	bool (*supported)(struct kvm_vcpu *vcpu);
 
+	/**
+	 * @init: Probe and initialize the feature on the vcpu
+	 *
+	 * This callback is optional. If provided, it will be called during
+	 * vcpu initialization to probe the feature availability and perform
+	 * any necessary initialization. Returns true if the feature is supported
+	 * and initialized successfully, false otherwise.
+	 */
+	bool (*init)(struct kvm_vcpu *vcpu);
+
 	/**
 	 * @reset: Reset the feature value irrespective whether feature is supported or not
 	 *
@@ -131,19 +141,30 @@ static long kvm_sbi_fwft_get_misaligned_delegation(struct kvm_vcpu *vcpu,
 
 static bool try_to_set_pmm(unsigned long value)
 {
+	unsigned long prev;
+	bool ret;
+
+	prev = csr_read_clear(CSR_HENVCFG, ENVCFG_PMM);
 	csr_set(CSR_HENVCFG, value);
-	return (csr_read_clear(CSR_HENVCFG, ENVCFG_PMM) & ENVCFG_PMM) == value;
+	ret = (csr_read_clear(CSR_HENVCFG, ENVCFG_PMM) & ENVCFG_PMM) == value;
+	csr_write(CSR_HENVCFG, prev);
+
+	return ret;
 }
 
 static bool kvm_sbi_fwft_pointer_masking_pmlen_supported(struct kvm_vcpu *vcpu)
 {
-	struct kvm_sbi_fwft *fwft = vcpu_to_fwft(vcpu);
+	return riscv_isa_extension_available(vcpu->arch.isa, SMNPM);
+}
 
-	if (!riscv_isa_extension_available(vcpu->arch.isa, SMNPM))
-		return false;
+static bool kvm_sbi_fwft_pointer_masking_pmlen_init(struct kvm_vcpu *vcpu)
+{
+	struct kvm_sbi_fwft *fwft = vcpu_to_fwft(vcpu);
 
+	preempt_disable();
 	fwft->have_vs_pmlen_7 = try_to_set_pmm(ENVCFG_PMM_PMLEN_7);
 	fwft->have_vs_pmlen_16 = try_to_set_pmm(ENVCFG_PMM_PMLEN_16);
+	preempt_enable();
 
 	return fwft->have_vs_pmlen_7 || fwft->have_vs_pmlen_16;
 }
@@ -231,6 +252,7 @@ static const struct kvm_sbi_fwft_feature features[] = {
 		.first_reg_num = offsetof(struct kvm_riscv_sbi_fwft, pointer_masking.enable) /
 				 sizeof(unsigned long),
 		.supported = kvm_sbi_fwft_pointer_masking_pmlen_supported,
+		.init = kvm_sbi_fwft_pointer_masking_pmlen_init,
 		.reset = kvm_sbi_fwft_reset_pointer_masking_pmlen,
 		.set = kvm_sbi_fwft_set_pointer_masking_pmlen,
 		.get = kvm_sbi_fwft_get_pointer_masking_pmlen,
@@ -365,6 +387,9 @@ static int kvm_sbi_ext_fwft_init(struct kvm_vcpu *vcpu)
 		else
 			conf->supported = true;
 
+		if (conf->supported && feature->init)
+			conf->supported = feature->init(vcpu);
+
 		conf->enabled = conf->supported;
 		conf->feature = feature;
 	}

-- 
2.43.7