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

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Message-ID: <ZUuyVfdKZG44T1ba@google.com>
Date:   Wed, 8 Nov 2023 08:07:49 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.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 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.

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

> 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.