Re: [PATCH v11 19/23] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

[Mathias Krause]

On 04.07.25 10:49, Chao Gao wrote:
> From: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> 
> Expose CET features to guest if KVM/host can support them, clear CPUID
> feature bits if KVM/host cannot support.
> [...]

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.

Using the old test from [1] gives the following numbers (perf stat -r 5
ssdd 10 50000):

* grsec guest on linux-6.16-rc5 + cet patches:
  2.4647 +- 0.0706 seconds time elapsed  ( +-  2.86% )

* grsec guest on linux-6.16-rc5 + cet patches + CR4.CET guest-owned:
  1.5648 +- 0.0240 seconds time elapsed  ( +-  1.53% )

Not only is it ~35% faster, it's also more stable, less fluctuation due
to less VMEXITs, I believe.

Thanks,
Mathias

[1]
https://lore.kernel.org/kvm/20230322013731.102955-1-minipli@xxxxxxxxxxxxxx/From 14ef5d8b952744c46c32f16fea3b29184cde3e65 Mon Sep 17 00:00:00 2001
From: Mathias Krause <minipli@xxxxxxxxxxxxxx>
Date: Mon, 21 Jul 2025 13:45:55 +0200
Subject: [PATCH] KVM: VMX: Make CR4.CET a guest owned bit

There's no need to intercept changes of CR4.CET, make it a guest-owned
bit where possible.

This change is VMX-specific, as SVM has no such fine-grained control
register intercept control.

Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
---
 arch/x86/kvm/kvm_cache_regs.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
index 36a8786db291..8ddb01191d6f 100644
--- a/arch/x86/kvm/kvm_cache_regs.h
+++ b/arch/x86/kvm/kvm_cache_regs.h
@@ -7,7 +7,8 @@
 #define KVM_POSSIBLE_CR0_GUEST_BITS	(X86_CR0_TS | X86_CR0_WP)
 #define KVM_POSSIBLE_CR4_GUEST_BITS				  \
 	(X86_CR4_PVI | X86_CR4_DE | X86_CR4_PCE | X86_CR4_OSFXSR  \
-	 | X86_CR4_OSXMMEXCPT | X86_CR4_PGE | X86_CR4_TSD | X86_CR4_FSGSBASE)
+	 | X86_CR4_OSXMMEXCPT | X86_CR4_PGE | X86_CR4_TSD | X86_CR4_FSGSBASE \
+	 | X86_CR4_CET)
 
 #define X86_CR0_PDPTR_BITS    (X86_CR0_CD | X86_CR0_NW | X86_CR0_PG)
 #define X86_CR4_TLBFLUSH_BITS (X86_CR4_PGE | X86_CR4_PCIDE | X86_CR4_PAE | X86_CR4_SMEP)
-- 
2.47.2