On Mon, Apr 22, 2024 at 10:12:53AM -0300, Daniel Henrique Barboza wrote:
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
^ the
riscv,isa. The guest kernel will be unaware of the extension, i.e. it
^ the
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.
I'd probably add a sentence or two explaining why we still want to warn,
which is because KVM is stating that while you may think you're disabling
an extension, that extension's behavior (instructions, etc.) will still
be exposed to the guest since there's no way not to expose it. The user
should be aware that a guest could use it anyway, since that means the
attempt to disable it can't be relied upon for migration compatibility of
arbitrary guests. The guest needs to be well behaved, i.e. one that won't
try to use any extensions which aren't in the ISA string. So, disabling
these types of extensions either shouldn't generally be done (so a noisy
warning helps prohibit that) or done for debug purposes (where a noisy
warning is fine).
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, ®);
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 {
Maybe we should check for a specific error. Is it EINVAL? If we do, then
the message below would drop the (error %d) part and instead state
explicitly that it cannot disable this extension since it's not possible.