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From: Sean Christopherson <seanjc@google.com>
To: Mathias Krause <minipli@grsecurity.net>
Cc: Chao Gao <chao.gao@intel.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	 x86@kernel.org, pbonzini@redhat.com, dave.hansen@intel.com,
	 rick.p.edgecombe@intel.com, mlevitsk@redhat.com,
	john.allen@amd.com,  weijiang.yang@intel.com, xin@zytor.com,
	Thomas Gleixner <tglx@linutronix.de>,
	 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	 Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v11 19/23] KVM: x86: Enable CET virtualization for VMX and advertise to userspace
Date: Tue, 22 Jul 2025 07:13:44 -0700	[thread overview]
Message-ID: <aH-cmDRPPp2X7OxN@google.com> (raw)
In-Reply-To: <96a0a8b2-3ebd-466c-9c6e-8ba63cd4e2e3@grsecurity.net>

On Tue, Jul 22, 2025, Mathias Krause wrote:
> On 21.07.25 19:45, Sean Christopherson wrote:
> > On Mon, Jul 21, 2025, Mathias Krause wrote:
> >> Can we please make CR4.CET a guest-owned bit as well (sending a patch in
> >> a second)? It's a logical continuation to making CR0.WP a guest-owned
> >> bit just that it's even easier this time, as no MMU role bits are
> >> involved and it still makes a big difference, at least for grsecurity
> >> guest kernels.
> > 
> > Out of curiosity, what's the use case for toggling CR4.CET at runtime?
> 
> Plain and simple: architectural requirements to be able to toggle CR0.WP.

Ugh, right.  That was less fun than I as expecting :-)

> > E.g. at one point CR4.LA57 was a guest-owned bit, and the code was buggy.  Fixing
> > things took far more effort than it should have there was no justification for
> > the logic (IIRC, it was done purely on the whims of the original developer).
> > 
> > KVM has had many such cases, where some weird behavior was never documented/justified,
> > and I really, really want to avoid committing the same sins that have caused me
> > so much pain :-)
> 
> I totally understand your reasoning, "just because" shouldn't be the
> justification. In this case, however, not making it a guest-owned bit
> has a big performance impact for grsecurity, we would like to address.

Oh, I'm not objecting to the change, at all.  I just want to make sure we capture
the justification in the changelog.

  reply	other threads:[~2025-07-22 14:13 UTC|newest]

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-07-04  8:49 [PATCH v11 00/23] Enable CET Virtualization Chao Gao
2025-07-04  8:49 ` [PATCH v11 01/23] KVM: x86: Rename kvm_{g,s}et_msr()* to show that they emulate guest accesses Chao Gao
2025-07-24 11:37   ` Huang, Kai
2025-07-24 13:31     ` Sean Christopherson
2025-07-28 22:31   ` Xin Li
2025-07-29  0:45     ` Chao Gao
2025-07-29 18:19       ` Sean Christopherson
2025-07-04  8:49 ` [PATCH v11 02/23] KVM: x86: Add kvm_msr_{read,write}() helpers Chao Gao
2025-07-04  8:49 ` [PATCH v11 03/23] KVM: x86: Manually clear MPX state only on INIT Chao Gao
2025-07-04  8:49 ` [PATCH v11 04/23] KVM: x86: Zero XSTATE components on INIT by iterating over supported features Chao Gao
2025-07-04  8:49 ` [PATCH v11 05/23] KVM: x86: Introduce KVM_{G,S}ET_ONE_REG uAPIs support Chao Gao
2025-07-04  8:49 ` [PATCH v11 06/23] KVM: x86: Report XSS as to-be-saved if there are supported features Chao Gao
2025-07-04  8:49 ` [PATCH v11 07/23] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS Chao Gao
2025-07-04  8:49 ` [PATCH v11 08/23] KVM: x86: Initialize kvm_caps.supported_xss Chao Gao
2025-07-04  8:49 ` [PATCH v11 09/23] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs Chao Gao
2025-07-04  8:49 ` [PATCH v11 10/23] KVM: x86: Add fault checks for guest CR4.CET setting Chao Gao
2025-07-04  8:49 ` [PATCH v11 11/23] KVM: x86: Report KVM supported CET MSRs as to-be-saved Chao Gao
2025-07-04  8:49 ` [PATCH v11 12/23] KVM: VMX: Introduce CET VMCS fields and control bits Chao Gao
2025-07-28 22:53   ` Xin Li
2025-07-29  1:30     ` Chao Gao
2025-07-29  2:17       ` Xin Li
2025-07-04  8:49 ` [PATCH v11 13/23] KVM: x86: Enable guest SSP read/write interface with new uAPIs Chao Gao
2025-07-04  8:49 ` [PATCH v11 14/23] KVM: VMX: Emulate read and write to CET MSRs Chao Gao
2025-07-04  8:49 ` [PATCH v11 15/23] KVM: x86: Save and reload SSP to/from SMRAM Chao Gao
2025-07-04  8:49 ` [PATCH v11 16/23] KVM: VMX: Set up interception for CET MSRs Chao Gao
2025-07-04  8:49 ` [PATCH v11 17/23] KVM: VMX: Set host constant supervisor states to VMCS fields Chao Gao
2025-07-04  8:49 ` [PATCH v11 18/23] KVM: x86: Don't emulate instructions guarded by CET Chao Gao
2025-07-04  8:49 ` [PATCH v11 19/23] KVM: x86: Enable CET virtualization for VMX and advertise to userspace Chao Gao
2025-07-21 15:51   ` Mathias Krause
2025-07-21 17:45     ` Sean Christopherson
2025-07-22  5:49       ` Mathias Krause
2025-07-22 14:13         ` Sean Christopherson [this message]
2025-07-22 21:25         ` [PATCH v2] KVM: VMX: Make CR4.CET a guest owned bit Mathias Krause
2025-07-23  6:24           ` Chao Gao
2025-08-06 20:58   ` [PATCH v11 19/23] KVM: x86: Enable CET virtualization for VMX and advertise to userspace John Allen
2025-08-06 22:47     ` Sean Christopherson
2025-07-04  8:49 ` [PATCH v11 20/23] KVM: nVMX: Virtualize NO_HW_ERROR_CODE_CC for L1 event injection to L2 Chao Gao
2025-07-04  8:49 ` [PATCH v11 21/23] KVM: nVMX: Enable CET support for nested guest Chao Gao
2025-07-28  6:30   ` Xin Li
2025-07-28  8:42     ` Chao Gao
2025-07-04  8:49 ` [PATCH v11 22/23] KVM: nVMX: Add consistency checks for CR0.WP and CR4.CET Chao Gao
2025-07-04  8:49 ` [PATCH v11 23/23] KVM: nVMX: Add consistency checks for CET states Chao Gao
2025-07-06 16:51 ` [PATCH v11 00/23] Enable CET Virtualization Xiaoyao Li
2025-07-07  1:32   ` Chao Gao
2025-07-16 20:36     ` John Allen
2025-07-17  7:00       ` Mathias Krause
2025-07-17  7:57         ` Chao Gao
2025-07-21 18:08           ` John Allen
2025-07-21 15:35 ` Mathias Krause

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