On 11/30/2023 3:22 AM, Sean Christopherson wrote:
On Mon, Nov 13, 2023, Xiaoyao Li wrote:
On 11/9/2023 12:07 AM, Sean Christopherson wrote:
On Wed, Nov 08, 2023, Xiaoyao Li wrote:
On 11/8/2023 9:09 AM, Sean Christopherson wrote:
Add yet another macro to the VM/vCPU ioctl() framework to detect when an
ioctl() failed because KVM killed/bugged the VM, i.e. when there was
nothing wrong with the ioctl() itself. If KVM kills a VM, e.g. by way of
a failed KVM_BUG_ON(), all subsequent VM and vCPU ioctl()s will fail with
-EIO, which can be quite misleading and ultimately waste user/developer
time.
Use KVM_CHECK_EXTENSION on KVM_CAP_USER_MEMORY to detect if the VM is
dead and/or bug, as KVM doesn't provide a dedicated ioctl(). Using a
heuristic is obviously less than ideal, but practically speaking the logic
is bulletproof barring a KVM change, and any such change would arguably
break userspace, e.g. if KVM returns something other than -EIO.
We hit similar issue when testing TDX VMs. Most failure of SEMCALL is
handled with a KVM_BUG_ON(), which leads to vm dead. Then the following
IOCTL from userspace (QEMU) and gets -EIO.
Can we return a new KVM_EXIT_VM_DEAD on KVM_REQ_VM_DEAD?
Why? Even if KVM_EXIT_VM_DEAD somehow provided enough information to be useful
from an automation perspective, the VM is obviously dead. I don't see how the
VMM can do anything but log the error and tear down the VM. KVM_BUG_ON() comes
with a WARN, which will be far more helpful for a human debugger, e.g. because
all vCPUs would exit with KVM_EXIT_VM_DEAD, it wouldn't even identify which vCPU
initially triggered the issue.
It's not about providing more helpful debugging info, but to provide a
dedicated notification for VMM that "the VM is dead, all the following
command may not response". With it, VMM can get rid of the tricky detection
like this patch.
But a VMM doesn't need this tricky detection, because this tricky detections isn't
about detecting that the VM is dead, it's all about helping a human debug why a
test failed.
-EIO already effectively says "the VM is dead", e.g. QEMU isn't going to keep trying
to run vCPUs.
If -EIO for KVM ioctl denotes "the VM is dead" is to be the officially
announced API, I'm fine.
Similarly, selftests assert either way, the goal is purely to print
out a unique error message to minimize the chances of confusing the human running
the test (or looking at results).
Definitely a "no" on this one. As has been established by the guest_memfd series,
it's ok to return -1/errno with a valid exit_reason.
But I'm wondering if any userspace relies on -EIO behavior for VM DEAD case.
I doubt userspace relies on -EIO, but userpsace definitely relies on -1/errno being
returned when a fatal error.
what about KVM_EXIT_SHUTDOWN? Or KVM_EXIT_INTERNAL_ERROR?
I don't follow,
I was trying to ask if KVM_EXIT_SHUTDOWN and KVM_EXIT_INTERNAL_ERROR are
treated as fatal error by userspace.
those are vcpu_run.exit_reason values, not errno values. Returning
any flavor of KVM_EXIT_*, which are positive values, would break userspace, e.g.
QEMU explicitly looks for "ret < 0", and glibc only treats small-ish negative
values as errors, i.e. a postive return value will be propagated verbatim up to
QEMU.
