[PATCH] RISC-V: KVM: Introduce KVM_EXIT_FAIL_ENTRY_NO_VSFILE

[BillXiang]

Consider a system with 8 harts, where each hart supports 5
Guest Interrupt Files (GIFs), yielding 40 total GIFs.
If we launch a QEMU guest with over 5 vCPUs using
"-M virt,aia='aplic-imsic' -accel kvm,riscv-aia=hwaccel" – which
relies solely on VS-files (not SW-files) for higher performance – the
guest requires more than 5 GIFs. However, the current Linux scheduler
lacks GIF awareness, potentially scheduling >5 vCPUs to a single hart.
This triggers VS-file allocation failure, and since no handler exists
for this error, the QEMU guest becomes corrupted.

To address this, we introduce KVM_EXIT_FAIL_ENTRY_NO_VSFILE upon
VS-file allocation failure. This provides an opportunity for graceful
error handling instead of corruption. For example, QEMU can handle
this exit by rescheduling vCPUs to alternative harts when VS-file
allocation fails on the current hart [1].

[1] https://github.com/BillXiang/qemu/tree/riscv-vsfile-alloc/

Signed-off-by: BillXiang <xiangwencheng at lanxincomputing.com>
---
 arch/riscv/include/uapi/asm/kvm.h | 2 ++
 arch/riscv/kvm/aia_imsic.c        | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/uapi/asm/kvm.h b/arch/riscv/include/uapi/asm/kvm.h
index 5f59fd226cc5..be29c3502fe4 100644
--- a/arch/riscv/include/uapi/asm/kvm.h
+++ b/arch/riscv/include/uapi/asm/kvm.h
@@ -22,6 +22,8 @@
 #define KVM_INTERRUPT_SET	-1U
 #define KVM_INTERRUPT_UNSET	-2U
 
+#define KVM_EXIT_FAIL_ENTRY_NO_VSFILE	(1ULL << 0)
+
 /* for KVM_GET_REGS and KVM_SET_REGS */
 struct kvm_regs {
 };
diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
index 29ef9c2133a9..69b0ab651389 100644
--- a/arch/riscv/kvm/aia_imsic.c
+++ b/arch/riscv/kvm/aia_imsic.c
@@ -760,7 +760,7 @@ int kvm_riscv_vcpu_aia_imsic_update(struct kvm_vcpu *vcpu)
 		/* For HW acceleration mode, we can't continue */
 		if (kvm->arch.aia.mode == KVM_DEV_RISCV_AIA_MODE_HWACCEL) {
 			run->fail_entry.hardware_entry_failure_reason =
-								CSR_HSTATUS;
+								KVM_EXIT_FAIL_ENTRY_NO_VSFILE;
 			run->fail_entry.cpu = vcpu->cpu;
 			run->exit_reason = KVM_EXIT_FAIL_ENTRY;
 			return 0;
-- 
2.46.2.windows.1