From: Sean Christopherson <seanjc@google.com>
To: Oliver Upton <oupton@google.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
David Dunn <daviddunn@google.com>
Subject: Re: [PATCH] KVM: x86: Introduce KVM_CAP_DISABLE_QUIRKS2
Date: Mon, 28 Feb 2022 18:22:02 +0000 [thread overview]
Message-ID: <Yh0SyrFvu6UBwsj3@google.com> (raw)
In-Reply-To: <Yh0NyuuzIymt9mgt@google.com>
On Mon, Feb 28, 2022, Oliver Upton wrote:
> On Mon, Feb 28, 2022 at 04:33:57PM +0000, Sean Christopherson wrote:
> > On Sat, Feb 26, 2022, Oliver Upton wrote:
> > > KVM_CAP_DISABLE_QUIRKS is irrevocably broken. The capability does not
> > > advertise the set of quirks which may be disabled to userspace, so it is
> > > impossible to predict the behavior of KVM. Worse yet,
> > > KVM_CAP_DISABLE_QUIRKS will tolerate any value for cap->args[0], meaning
> > > it fails to reject attempts to set invalid quirk bits.
> >
> > FWIW, we do have a way out without adding another capability. The 'flags' field
> > is enforced for all capabilities, we could use a bit there to add "v2" functionality.
> > Userspace can assume KVM_QUIRK_ENFORCE_QUIRKS is allowed if the return from probing
> > the capability is >1.
> >
> > It's gross and forced, just an idea if we want to avoid yet another cap.
>
> I had considered this before sending out v1, but was concerned if a
> userspace didn't correctly handle a return value >1 from
> KVM_CHECK_EXTENSION.
Ah, right, userspace could theoretically check "== 1". Blech.
> Turns out, I can't even find any evidence of the KVM_CAP_DISABLE_QUIRKS used
> by userspace. I spot checked QEMU, kvmtool,
> and a couple of the rusty ones.
>
> The only other thing that comes to mind is it's a bit gross for userspace
> to do a graceful fallback if KVM_QUIRK_ENFORCE_QUIRKS isn't valid, since
> most userspace would just error out on -EINVAL. At least with a new cap
> userspace could follow a somewhat standardized way to discover if the
> kernel supports enforced quirks.
Yeah, a QUIRKS2 is probably easier for everyone.
prev parent reply other threads:[~2022-02-28 18:35 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-26 0:21 [PATCH] KVM: x86: Introduce KVM_CAP_DISABLE_QUIRKS2 Oliver Upton
2022-02-26 0:22 ` Oliver Upton
2022-02-28 16:33 ` Sean Christopherson
2022-02-28 18:00 ` Oliver Upton
2022-02-28 18:22 ` Sean Christopherson [this message]
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