From: Sean Christopherson <seanjc@google.com>
To: Yu Zhang <yu.c.zhang@linux.intel.com>
Cc: Hou Wenlong <houwenlong93@linux.alibaba.com>,
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>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Jan Kiszka <jan.kiszka@siemens.com>, Avi Kivity <avi@redhat.com>,
"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 1/3] kvm: x86: Introduce hypercall x86 ops for handling hypercall not in cpl0
Date: Thu, 9 Sep 2021 17:09:11 +0000 [thread overview]
Message-ID: <YTo/t4G1iI28oDmk@google.com> (raw)
In-Reply-To: <20210909163901.2vvozmkuxjcgabs5@linux.intel.com>
On Fri, Sep 10, 2021, Yu Zhang wrote:
> On Thu, Sep 09, 2021 at 07:55:23PM +0800, Hou Wenlong wrote:
> > Per Intel's SDM, use vmcall instruction in non VMX operation for cpl3
> > it should trigger a #UD. And in VMX root operation, it should
>
> Are you sure? IIRC, vmcall will always cause VM exit as long as CPU
> is in non-root mode(regardless the CPL).
Correct, VMCALL unconditionally causes VM-Exit in non-root mode, but Hou is
referring to the first fault condition of "non VMX operation". The intent of the
patch is to emulate hardware behavior for CPL>0: if L1 is not in VMX operation,
a.k.a. not post-VMXON, then #UD, else #GP (because VMCALL #GPs at CPL>0 in VMX
root).
On one hand, I agree with Hou's logic; injecting #UD/#GP is architecturally
correct if KVM is emulating a bare metal environment for the guest. On the
other hand, that contradicts with KVM _not_ injecting #UD for guest CPL0, i.e.
KVM is clearly not emulating a bare metal environment.
In the end, this would represent an ABI change for guest CPL>0. While it's highly
unlikely that such a change would cause problems, maintaining the current behavior
is the safe option unless there's strong motivation for changing the guest ABI.
And injecting #UD/#GP would also mean KVM would again have to change its ABI if
there is a future hypercall KVM wants to allow at CPL>0. Again, that's unlikely,
but again I don't see sufficient justification.
next prev parent reply other threads:[~2021-09-09 17:09 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-09 11:37 [PATCH 0/3] some fixes of hypercall emulation Hou Wenlong
2021-09-09 11:37 ` [PATCH 1/3] kvm: x86: Introduce hypercall x86 ops for handling hypercall not in cpl0 Hou Wenlong
2021-09-09 11:37 ` [PATCH 2/3] kvm: x86: Refactor kvm_emulate_hypercall() to no skip instruction Hou Wenlong
2021-09-09 11:37 ` [PATCH 3/3] kvm: x86: Emulate hypercall instead of fixing hypercall instruction Hou Wenlong
2021-09-09 11:55 ` [PATCH v2 0/3] kvm: x86: some fixes of hypercall emulation Hou Wenlong
2021-09-09 11:55 ` [PATCH v2 1/3] kvm: x86: Introduce hypercall x86 ops for handling hypercall not in cpl0 Hou Wenlong
2021-09-09 16:39 ` Yu Zhang
2021-09-09 17:09 ` Sean Christopherson [this message]
2021-09-10 1:53 ` Yu Zhang
2021-09-09 11:55 ` [PATCH v2 2/3] kvm: x86: Refactor kvm_emulate_hypercall() to no skip instruction Hou Wenlong
2021-09-09 11:55 ` [PATCH v2 3/3] kvm: x86: Emulate hypercall instead of fixing hypercall instruction Hou Wenlong
2021-09-16 16:00 ` Sean Christopherson
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