Re: [PATCH v3 04/10] KVM: x86: Honor KVM_GUESTDBG_USE_HW_BP when emulating MOV DR (in emulator)

[Yosry Ahmed <yosry@kernel.org>]

On Fri, May 15, 2026 at 03:26:32PM -0700, Sean Christopherson wrote:
> From: Hou Wenlong <houwenlong.hwl@antgroup.com>
> 
> When emulating a MOV DR instruction, honor KVM_GUESTDBG_USE_HW_BP when
> checking DR7.GD, and if there is a general-detect #DB, route it to host
> userspace as appropriate.  Consulting only the guest's actual DR7 causes
> KVM to fail to report a DR access to userspace (assuming the guest itself
> doesn't have DR7.GD=1).
> 
> Fixes: ae675ef01cd8 ("KVM: x86: Wire-up hardware breakpoints for guest debugging")
> Suggested-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> [sean: only expose effective DR7 to emulator, massage changelog]
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/emulate.c     |  2 +-
>  arch/x86/kvm/kvm_emulate.h |  1 +
>  arch/x86/kvm/x86.c         | 41 ++++++++++++++++++++++++++++++--------
>  3 files changed, 35 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 510244555a74..917a521c299f 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -3848,7 +3848,7 @@ static int check_dr_read(struct x86_emulate_ctxt *ctxt)
>  	if ((cr4 & X86_CR4_DE) && (dr == 4 || dr == 5))
>  		return emulate_ud(ctxt);
>  
> -	if (ctxt->ops->get_dr(ctxt, 7) & DR7_GD)
> +	if (ctxt->ops->get_eff_dr7(ctxt) & DR7_GD)
>  		return emulate_db(ctxt, DR6_BD);
>  
>  	return X86EMUL_CONTINUE;
> diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
> index bb2a2aee0e13..33bfc9aa948e 100644
> --- a/arch/x86/kvm/kvm_emulate.h
> +++ b/arch/x86/kvm/kvm_emulate.h
> @@ -215,6 +215,7 @@ struct x86_emulate_ops {
>  	ulong (*get_cr)(struct x86_emulate_ctxt *ctxt, int cr);
>  	int (*set_cr)(struct x86_emulate_ctxt *ctxt, int cr, ulong val);
>  	int (*cpl)(struct x86_emulate_ctxt *ctxt);
> +	ulong (*get_eff_dr7)(struct x86_emulate_ctxt *ctxt);

I would name this get_effective_dr7, and the callback
kvm_get_effective_dr7(). I don't think saving a few characters here
offsets the readability losses.