This is a note to let you know that I've just added the patch titled
KVM: nVMX: Emulate NOPs in L2, and PAUSE if it's not intercepted
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kvm-nvmx-emulate-nops-in-l2-and-pause-if-it-s-not-intercepted.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 4984563823f0034d3533854c1b50e729f5191089 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@xxxxxxxxxx>
Date: Tue, 4 Apr 2023 17:23:59 -0700
Subject: KVM: nVMX: Emulate NOPs in L2, and PAUSE if it's not intercepted
From: Sean Christopherson <seanjc@xxxxxxxxxx>
commit 4984563823f0034d3533854c1b50e729f5191089 upstream.
Extend VMX's nested intercept logic for emulated instructions to handle
"pause" interception, in quotes because KVM's emulator doesn't filter out
NOPs when checking for nested intercepts. Failure to allow emulation of
NOPs results in KVM injecting a #UD into L2 on any NOP that collides with
the emulator's definition of PAUSE, i.e. on all single-byte NOPs.
For PAUSE itself, honor L1's PAUSE-exiting control, but ignore PLE to
avoid unnecessarily injecting a #UD into L2. Per the SDM, the first
execution of PAUSE after VM-Entry is treated as the beginning of a new
loop, i.e. will never trigger a PLE VM-Exit, and so L1 can't expect any
given execution of PAUSE to deterministically exit.
... the processor considers this execution to be the first execution of
PAUSE in a loop. (It also does so for the first execution of PAUSE at
CPL 0 after VM entry.)
All that said, the PLE side of things is currently a moot point, as KVM
doesn't expose PLE to L1.
Note, vmx_check_intercept() is still wildly broken when L1 wants to
intercept an instruction, as KVM injects a #UD instead of synthesizing a
nested VM-Exit. That issue extends far beyond NOP/PAUSE and needs far
more effort to fix, i.e. is a problem for the future.
Fixes: 07721feee46b ("KVM: nVMX: Don't emulate instructions in guest mode")
Cc: Mathias Krause <minipli@xxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230405002359.418138-1-seanjc@xxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kvm/vmx/vmx.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7266,6 +7266,21 @@ static int vmx_check_intercept(struct kv
/* FIXME: produce nested vmexit and return X86EMUL_INTERCEPTED. */
break;
+ case x86_intercept_pause:
+ /*
+ * PAUSE is a single-byte NOP with a REPE prefix, i.e. collides
+ * with vanilla NOPs in the emulator. Apply the interception
+ * check only to actual PAUSE instructions. Don't check
+ * PAUSE-loop-exiting, software can't expect a given PAUSE to
+ * exit, i.e. KVM is within its rights to allow L2 to execute
+ * the PAUSE.
+ */
+ if ((info->rep_prefix != REPE_PREFIX) ||
+ !nested_cpu_has2(vmcs12, CPU_BASED_PAUSE_EXITING))
+ return X86EMUL_CONTINUE;
+
+ break;
+
/* TODO: check more intercepts... */
default:
break;
Patches currently in stable-queue which might be from seanjc@xxxxxxxxxx are
queue-5.4/kvm-nvmx-emulate-nops-in-l2-and-pause-if-it-s-not-intercepted.patch
