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: <ZWiztxAzjCAUw7cx@google.com>
Date:   Thu, 30 Nov 2023 08:09:27 -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 Thu, Nov 30, 2023, Xiaoyao Li wrote:
> 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.

Yes, -EIO is effectively ABI at this point.  Though there is the caveat that -EIO
doesn't guarantee KVM killed the VM, i.e. KVM could return -EIO for some other
reason (though that's highly unlikely for KVM_RUN at least).

> > 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.

Ah.  Not really.  SHUTDOWN isn't fatal per se, e.g. QEMU emulates a RESET if a
vCPU hits shutdown.  INTERNAL_ERROR isn't always fatal on x86, e.g. QEMU ignores
(I think that's what happens) emulation failure when the vCPU is at CPL > 0 so
that guest userspace can't DoS the VM.