[PATCH v2] RISC-V: KVM: Fix spurious -EEXIST and clean up gstage fault path types

[Bingyu.Xian]

Two small fixes to the RISC-V G-stage page fault path, both suggested
during review of the in-progress KVM Userfault port.

1. Treat -EEXIST from kvm_riscv_gstage_map_page() as a quiet success.

   When a concurrent vCPU installs the same G-stage mapping while we
   are waiting for mmu_lock, gstage_map_page() returns -EEXIST.  This
   is not an error -- the page is correctly mapped and the faulting
   vCPU can simply retry the guest instruction -- but KVM was treating
   it as one: printing "Failed to map in G-stage" to dmesg and
   propagating -EEXIST all the way to userspace.

   Align RISC-V with x86 and arm64, which already swallow -EEXIST in
   their respective fault handlers.  This also lets
   kvm_release_faultin_page() drop its "ret && ret != -EEXIST" special
   case: with ret normalized to 0 the regular release path is correct.

2. Clean up fault path types.

   - vma_pageshift: short -> unsigned int.  Bit widths and shift counts
     conventionally use unsigned int in the kernel.

   - fault_addr: unsigned long -> gpa_t.  RV32 with Sv32x4 has 34-bit
     guest physical addresses.  fault_addr is widened to gpa_t (u64) to
     hold the full address, and the shift reconstructing it,
     (trap->htval << 2), is cast to gpa_t before the shift: htval is
     unsigned long, so on RV32 the shift would otherwise be evaluated in
     32-bit arithmetic and drop bits 32/33 before the result is widened.
     No functional change on RV64.

   These type changes are in preparation for sharing a common
   struct kvm_page_fault across architectures, as requested during
   review.

No functional change on RV64 beyond silencing the spurious -EEXIST.

These are independent fixes with no dependencies; they can be merged
on their own.  The KVM_MEM_USERFAULT port that motivated them will be
sent separately as an RFC once the generic userfault series lands.

Fixes: 9d05c1fee837 ("RISC-V: KVM: Implement stage2 page table programming")
Assisted-by: YuanSheng: deepseek-v4-pro
Co-developed-by: Quan Zhou <zhouquan at iscas.ac.cn>
Signed-off-by: Quan Zhou <zhouquan at iscas.ac.cn>
Signed-off-by: Bingyu Xian <shanbeeyoo at gmail.com>
---
Changes since v1:
- Cast trap->htval to gpa_t before the <<2 shift so the 34-bit guest
  physical address is not truncated by 32-bit arithmetic on RV32.  v1
  only widened the destination (fault_addr -> gpa_t); the shift itself
  still dropped bits 32/33 before the result was widened.

 arch/riscv/kvm/mmu.c       | 8 +++++---
 arch/riscv/kvm/vcpu_exit.c | 5 +++--
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
index 8a0aa5e0e216..d2a06a54be17 100644
--- a/arch/riscv/kvm/mmu.c
+++ b/arch/riscv/kvm/mmu.c
@@ -541,7 +541,7 @@ int kvm_riscv_mmu_map(struct kvm_vcpu *vcpu, struct kvm_memory_slot *memslot,
 	kvm_pfn_t hfn;
 	bool is_hugetlb;
 	bool writable;
-	short vma_pageshift;
+	unsigned int vma_pageshift;
 	gfn_t gfn = gpa >> PAGE_SHIFT;
 	struct vm_area_struct *vma;
 	struct kvm *kvm = vcpu->kvm;
@@ -652,11 +652,13 @@ int kvm_riscv_mmu_map(struct kvm_vcpu *vcpu, struct kvm_memory_slot *memslot,
 						vma_pagesize, true, true, out_map);
 	}
 
-	if (ret)
+	if (ret == -EEXIST)
+		ret = 0;
+	else if (ret)
 		kvm_err("Failed to map in G-stage\n");
 
 out_unlock:
-	kvm_release_faultin_page(kvm, page, ret && ret != -EEXIST, writable);
+	kvm_release_faultin_page(kvm, page, ret, writable);
 	write_unlock(&kvm->mmu_lock);
 	return ret;
 }
diff --git a/arch/riscv/kvm/vcpu_exit.c b/arch/riscv/kvm/vcpu_exit.c
index 6c8530b9f29e..28cf9b27bb07 100644
--- a/arch/riscv/kvm/vcpu_exit.c
+++ b/arch/riscv/kvm/vcpu_exit.c
@@ -17,12 +17,13 @@ static int gstage_page_fault(struct kvm_vcpu *vcpu, struct kvm_run *run,
 {
 	struct kvm_gstage_mapping host_map;
 	struct kvm_memory_slot *memslot;
-	unsigned long hva, fault_addr;
+	unsigned long hva;
+	gpa_t fault_addr;
 	bool writable;
 	gfn_t gfn;
 	int ret;
 
-	fault_addr = (trap->htval << 2) | (trap->stval & 0x3);
+	fault_addr = ((gpa_t)trap->htval << 2) | (trap->stval & 0x3);
 	gfn = fault_addr >> PAGE_SHIFT;
 	memslot = gfn_to_memslot(vcpu->kvm, gfn);
 	hva = gfn_to_hva_memslot_prot(memslot, gfn, &writable);
-- 
2.54.0