[PATCH] LoongArch: KVM: return full old CSR value from kvm_emu_xchg_csr()

[Qiang Ma <maqianga@uniontech.com>]

The LoongArch CSRXCHG instruction returns the full old CSR value in rd
after applying the masked update. kvm_emu_xchg_csr() currently masks
the saved value before returning it to the guest, so rd receives only
the bits selected by the write mask.

That breaks the architectural behavior and makes a zero mask return 0
instead of the previous CSR value. Keep the masked CSR update, but
return the unmodified old CSR value.

Fixes: da50f5a693ff ("LoongArch: KVM: Implement handle csr exception")
Cc: stable@vger.kernel.org
Signed-off-by: Qiang Ma <maqianga@uniontech.com>
---
 arch/loongarch/kvm/exit.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/loongarch/kvm/exit.c b/arch/loongarch/kvm/exit.c
index 3b95cd0f989b..264813d45cbe 100644
--- a/arch/loongarch/kvm/exit.c
+++ b/arch/loongarch/kvm/exit.c
@@ -103,7 +103,6 @@ static unsigned long kvm_emu_xchg_csr(struct kvm_vcpu *vcpu, int csrid,
 		old = kvm_read_sw_gcsr(csr, csrid);
 		val = (old & ~csr_mask) | (val & csr_mask);
 		kvm_write_sw_gcsr(csr, csrid, val);
-		old = old & csr_mask;
 	} else
 		pr_warn_once("Unsupported csrxchg 0x%x with pc %lx\n", csrid, vcpu->arch.pc);
 
-- 
2.20.1