linux-kernel - Re: [PATCH] KVM: nVMX: set rflags to specify success in handle

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Date:   Thu, 23 Jan 2020 15:01:25 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        linmiaohe <linmiaohe@...wei.com>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] KVM: nVMX: set rflags to specify success in
 handle_invvpid() default case

On Thu, Jan 23, 2020 at 10:22:24AM -0800, Jim Mattson wrote:
> On Thu, Jan 23, 2020 at 1:54 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >
> > On 23/01/20 10:45, Vitaly Kuznetsov wrote:
> > >>> SDM says that "If an
> > >>> unsupported INVVPID type is specified, the instruction fails." and this
> > >>> is similar to INVEPT and I decided to check what handle_invept()
> > >>> does. Well, it does BUG_ON().
> > >>>
> > >>> Are we doing the right thing in any of these cases?
> > >>
> > >> Yes, both INVEPT and INVVPID catch this earlier.
> > >>
> > >> So I'm leaning towards not applying Miaohe's patch.
> > >
> > > Well, we may at least want to converge on BUG_ON() for both
> > > handle_invvpid()/handle_invept(), there's no need for them to differ.
> >
> > WARN_ON_ONCE + nested_vmx_failValid would probably be better, if we
> > really want to change this.
> >
> > Paolo
> 
> In both cases, something is seriously wrong. The only plausible
> explanations are compiler error or hardware failure. It would be nice
> to handle *all* such failures with a KVM_INTERNAL_ERROR exit to
> userspace. (I'm also thinking of situations like getting a VM-exit for
> INIT.)

Ya.  Vitaly and I had a similar discussion[*].  The idea we tossed around
was to also mark the VM as having encountered a KVM/hardware bug so that
the VM is effectively dead.  That would also allow gracefully handling bugs
that are detected deep in the stack, i.e. can't simply return 0 to get out
to userspace.

[*] https://lkml.kernel.org/r/20190930153358.GD14693@linux.intel.com