From: Sean Christopherson <sean.j.christopherson@intel.com>
To: stsp <stsp2@yandex.ru>
Cc: 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>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup
Date: Thu, 8 Oct 2020 21:04:54 -0700 [thread overview]
Message-ID: <20201009040453.GA10744@linux.intel.com> (raw)
In-Reply-To: <7efe1398-24c0-139f-29fa-3d89b6013f34@yandex.ru>
On Thu, Oct 08, 2020 at 09:18:18PM +0300, stsp wrote:
> 08.10.2020 20:59, Sean Christopherson пишет:
> >On Thu, Oct 08, 2020 at 07:00:13PM +0300, stsp wrote:
> >>07.10.2020 04:44, Sean Christopherson пишет:
> >>>Two bug fixes to handle KVM_SET_SREGS without a preceding KVM_SET_CPUID2.
> >>Hi Sean & KVM devs.
> >>
> >>I tested the patches, and wherever I
> >>set VMXE in CR4, I now get
> >>KVM: KVM_SET_SREGS: Invalid argument
> >>Before the patch I was able (with many
> >>problems, but still) to set VMXE sometimes.
> >>
> >>So its a NAK so far, waiting for an update. :)
> >IIRC, you said you were going to test on AMD? Assuming that's correct,
>
> Yes, that is true.
>
>
> > -EINVAL
> >is the expected behavior. KVM was essentially lying before; it never actually
> >set CR4.VMXE in hardware, it just didn't properply detect the error and so VMXE
> >was set in KVM's shadow of the guest's CR4.
>
> Hmm. But at least it was lying
> similarly on AMD and Intel CPUs. :)
> So I was able to reproduce the problems
> myself.
> Do you mean, any AMD tests are now useless, and we need to proceed with Intel
> tests only?
For anything VMXE related, yes.
> Then additional question.
> On old Intel CPUs we needed to set VMXE in guest to make it to work in
> nested-guest mode.
> Is it still needed even with your patches?
> Or the nested-guest mode will work now even on older Intel CPUs and KVM will
> set VMXE for us itself, when needed?
I'm struggling to even come up with a theory as to how setting VMXE from
userspace would have impacted KVM with unrestricted_guest=n, let alone fixed
anything.
CR4.VMXE must always be 1 in _hardware_ when VMX is on, including when running
the guest. But KVM forces vmcs.GUEST_CR4.VMXE=1 at all times, regardless of
the guest's actual value (the guest sees a shadow value when it reads CR4).
And unless I grossly misunderstand dosemu2, it's not doing anything related to
nested virtualization, i.e. the stuffing VMXE=1 for the guest's shadow value
should have absolutely zero impact.
More than likely, VMXE was a red herring. Given that the reporter is also
seeing the same bug on bare metal after moving to kernel 5.4, odds are good
the issue is related to unrestricted_guest=n and has nothing to do with nVMX.
next prev parent reply other threads:[~2020-10-09 4:05 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-07 1:44 [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup Sean Christopherson
2020-10-07 1:44 ` [PATCH 1/6] KVM: VMX: Drop guest CPUID check for VMXE in vmx_set_cr4() Sean Christopherson
2020-10-07 1:44 ` [PATCH 2/6] KVM: VMX: Drop explicit 'nested' check from vmx_set_cr4() Sean Christopherson
2020-10-07 1:44 ` [PATCH 3/6] KVM: SVM: Drop VMXE check from svm_set_cr4() Sean Christopherson
2020-10-07 1:44 ` [PATCH 4/6] KVM: x86: Move vendor CR4 validity check to dedicated kvm_x86_ops hook Sean Christopherson
2020-10-07 1:44 ` [PATCH 5/6] KVM: x86: Return bool instead of int for CR4 and SREGS validity checks Sean Christopherson
2020-10-07 1:44 ` [PATCH 6/6] KVM: selftests: Verify supported CR4 bits can be set before KVM_SET_CPUID2 Sean Christopherson
2020-10-08 16:00 ` [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup stsp
2020-10-08 17:59 ` Sean Christopherson
2020-10-08 18:18 ` stsp
2020-10-09 4:04 ` Sean Christopherson [this message]
2020-10-09 14:11 ` stsp
2020-10-09 15:30 ` Sean Christopherson
2020-10-09 15:48 ` stsp
2020-10-09 16:11 ` Sean Christopherson
2020-12-07 11:19 ` KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup) stsp
2020-12-07 11:24 ` stsp
2020-12-07 11:29 ` Paolo Bonzini
2020-12-07 11:47 ` stsp
[not found] ` <CABgObfYS57_ez-t=eu9+3S2bhSXC_9DTj=64Sna2jnYEMYo2Ag@mail.gmail.com>
2020-12-07 14:03 ` stsp
[not found] ` <CABgObfb_4r=k_qakd+48hPar8rzc-P50+dgdoYvQaL2H-po6+g@mail.gmail.com>
2020-12-07 14:29 ` stsp
[not found] ` <CABgObfYN7Okdt+YfHtsd3M_00iuWf=UyKPmbQhhYBhoiMtdXuw@mail.gmail.com>
2020-12-07 14:41 ` stsp
2020-12-07 23:59 ` Jim Mattson
2020-11-13 11:36 ` [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup Paolo Bonzini
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