linux-kernel - Re: [PATCH v2 2/2] KVM: selftests: Add logic to detect if ioctl() failed because VM was killed

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Date:   Mon, 13 Nov 2023 12:04:39 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Michal Luczaj <mhal@...x.co>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Colton Lewis <coltonlewis@...gle.com>
Subject: Re: [PATCH v2 2/2] KVM: selftests: Add logic to detect if ioctl()
 failed because VM was killed

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.

> Using an exit reason is a also bit tricky because it requires a vCPU, whereas a
> dead VM blocks anything and everything.

No argue of it. It cannot work for all the case, but at least it can 
make some case happier.

>> and replace -EIO with 0? yes, it's a ABI change.
> 
> 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?