From: Sean Christopherson <seanjc@google.com>
To: Jim Mattson <jmattson@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Shuah Khan <shuah@kernel.org>,
kvm@vger.kernel.org, linux-kselftest@vger.kernel.org,
linux-kernel@vger.kernel.org, Yosry Ahmed <yosry@kernel.org>
Subject: Re: [PATCH 1/3] KVM: SVM: Check EFER.SVME before updating MSR_VM_CR on guest writes
Date: Thu, 29 Oct 2026 14:52:23 -0700 [thread overview]
Message-ID: <auPAFxXy4ulcK_jq@google.com> (raw)
In-Reply-To: <CALMp9eTKXa9v1TxJMPrE-tSXajwJCErCk=g34dOPJtG057t5mg@mail.gmail.com>
On Thu, Oct 29, 2026, Jim Mattson wrote:
> On Thu, Oct 29, 2026 at 2:19 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Thu, Oct 29, 2026, Jim Mattson wrote:
> > > (Host- initiated writes are allowed to set any valid MSR_VM_CR bits.)
> >
> > Eww. Why? If we allow this, then we'll end up with the mess where the guest
> > can't write back the current value (if userspace is being weird). I get that
> > we want to avoid ordering issues, but there's *already* a required order here,
> > unless userspace ignores the failure from svm_set_vm_cr() and continues on.
> >
> > KVM doesn't advertise the MSR as something to be saved/restored, and none of
> > QEMU, CHV, CrosVM, or our VMM appear to save/restore it. So why make trouble
> > for ourselves?
>
> OMG! I didn't look any farther than the obviously broken svm_set_vm_cr().
>
> It looks like svm->nested.vm_cr_msr controls nothing other than itself.
LOL, nice.
> It's just 5 bits of storage with some weird locking rules, and it
> resets to 0 on suspend/resume.
>
> What is the point of such half-baked support?
More than likely, so that older kernels didn't crash when writing an MSR KVM
didn't emulate. :-(
next prev parent reply other threads:[~2026-10-29 21:52 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-29 20:28 [PATCH 1/3] KVM: SVM: Check EFER.SVME before updating MSR_VM_CR on guest writes Jim Mattson
2026-10-29 20:28 ` [PATCH 2/3] KVM: selftests: Rename svm_nested_efer_test to svm_msrs_test Jim Mattson
2026-10-29 20:28 ` [PATCH 3/3] KVM: selftests: Add MSR_VM_CR tests " Jim Mattson
2026-10-29 21:19 ` [PATCH 1/3] KVM: SVM: Check EFER.SVME before updating MSR_VM_CR on guest writes Sean Christopherson
2026-10-29 21:46 ` Jim Mattson
2026-10-29 21:52 ` Sean Christopherson [this message]
2026-10-29 22:04 ` Jim Mattson
2026-10-30 0:00 ` Jim Mattson
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