linux-kernel - Re: [PATCH 4/7] KVM: x86: Consolidate KVM_GUESTDBG_SINGLESTEP check into the kvm_inject_emulated

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Message-ID: <aTMdLPvT3gywUY6F@google.com>
Date: Fri, 5 Dec 2025 09:58:04 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Hou Wenlong <houwenlong.hwl@...group.com>
Cc: kvm@...r.kernel.org, Lai Jiangshan <jiangshan.ljs@...group.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] KVM: x86: Consolidate KVM_GUESTDBG_SINGLESTEP check
 into the kvm_inject_emulated_db()

On Wed, Sep 10, 2025, Hou Wenlong wrote:
> Use kvm_inject_emulated_db() in kvm_vcpu_do_singlestep() to consolidate
> 'KVM_GUESTDBG_SINGLESTEP' check into kvm_inject_emulated_db() during
> emulation.
> 
> No functional change intended.
> 
> Suggested-by: Lai Jiangshan <jiangshan.ljs@...group.com>
> Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
> ---
>  arch/x86/kvm/x86.c | 17 +++++------------
>  1 file changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5af652916a19..83960214d5d8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8632,7 +8632,10 @@ static int kvm_inject_emulated_db(struct kvm_vcpu *vcpu, unsigned long dr6)
>  {
>  	struct kvm_run *kvm_run = vcpu->run;
>  
> -	if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) {
> +	/* Data breakpoints are not supported in emulation for now. */
> +	WARN_ON((dr6 & DR6_BS) && (dr6 & DR_TRAP_BITS));

If we keep this, it should be a WARN_ON_ONCE().  We've had at least one case where
a sanity check in the emulator caused major problems because a WARN_ON() spammed
the kernel log to the point where it overloaded things :-)

But I think the WARN will be subject to false positives.  KVM doesn't emulate data
#DBs, but it does emulate code #DBs, and fault-like code #DBs can be coincident
with trap-like single-step #DBs.  Ah, but kvm_vcpu_check_code_breakpoint() doesn't
account for RFLAGS.TF.  That should probably be addressed in this series, especially
since it's consolidating KVM_GUESTDBG_SINGLESTEP handling.