[PATCH] target/riscv/kvm: tolerate KVM disable ext errors

[Daniel Henrique Barboza]

Running a KVM guest using a 6.9-rc3 kernel, in a 6.8 host that has zkr
enabled, will fail with a kernel oops SIGILL right at the start. The
reason is that we can't expose zkr without implementing the SEED CSR.
Disabling zkr in the guest would be a workaround, but if the KVM doesn't
allow it we'll error out and never boot.

In hindsight this is too strict. If we keep proceeding, despite not
disabling the extension in the KVM vcpu, we'll not add extension in
riscv,isa. The guest kernel will be unaware of the extension, i.e. it
doesn't matter if the KVM vcpu has it enabled underneath or not. So it's
ok to keep booting in this case.

Change our current logic to not error out if we fail to disable an
extension in kvm_set_one_reg(): throw an warning instead and keep
booting.  We'll keep the existing behavior when we fail to enable an
extension in KVM, since adding the extension in riscv,isa at this point
will cause a guest malfunction because the extension isn't enabled in
the vcpu.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
---
 target/riscv/kvm/kvm-cpu.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/target/riscv/kvm/kvm-cpu.c b/target/riscv/kvm/kvm-cpu.c
index 6a6c6cae80..261ca24504 100644
--- a/target/riscv/kvm/kvm-cpu.c
+++ b/target/riscv/kvm/kvm-cpu.c
@@ -427,10 +427,14 @@ static void kvm_riscv_update_cpu_cfg_isa_ext(RISCVCPU 
*cpu, CPUState *cs)
         reg = kvm_cpu_cfg_get(cpu, multi_ext_cfg);
         ret = kvm_set_one_reg(cs, id, &reg);
         if (ret != 0) {
-            error_report("Unable to %s extension %s in KVM, error %d",
-                         reg ? "enable" : "disable",
-                         multi_ext_cfg->name, ret);
-            exit(EXIT_FAILURE);
+            if (reg) {
+                error_report("Unable to enable extension %s in KVM, error %d",
+                             multi_ext_cfg->name, ret);
+                exit(EXIT_FAILURE);
+            } else {
+                warn_report("KVM did not disable extension %s (error %d)",
+                            multi_ext_cfg->name, ret);
+            }
         }
     }
 }
-- 
2.44.0