Re: [PATCH v3 10/16] KVM: arm64: Use debug_owner to track if debug regs need save/restore

[Marc Zyngier <maz@kernel.org>]

On Mon, 09 Dec 2024 18:09:20 +0000,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index f84306b11261..5b8982bcc0bb 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -621,40 +621,12 @@ static bool trap_dbgauthstatus_el1(struct kvm_vcpu *vcpu,
>  	}
>  }
>  
> -/*
> - * We want to avoid world-switching all the DBG registers all the
> - * time:
> - *
> - * - If we've touched any debug register, it is likely that we're
> - *   going to touch more of them. It then makes sense to disable the
> - *   traps and start doing the save/restore dance
> - * - If debug is active (DBG_MDSCR_KDE or DBG_MDSCR_MDE set), it is
> - *   then mandatory to save/restore the registers, as the guest
> - *   depends on them.
> - *
> - * For this, we use a DIRTY bit, indicating the guest has modified the
> - * debug registers, used as follow:
> - *
> - * On guest entry:
> - * - If the dirty bit is set (because we're coming back from trapping),
> - *   disable the traps, save host registers, restore guest registers.
> - * - If debug is actively in use (DBG_MDSCR_KDE or DBG_MDSCR_MDE set),
> - *   set the dirty bit, disable the traps, save host registers,
> - *   restore guest registers.
> - * - Otherwise, enable the traps
> - *
> - * On guest exit:
> - * - If the dirty bit is set, save guest registers, restore host
> - *   registers and clear the dirty bit. This ensure that the host can
> - *   now use the debug registers.
> - */
>  static bool trap_debug_regs(struct kvm_vcpu *vcpu,
>  			    struct sys_reg_params *p,
>  			    const struct sys_reg_desc *r)
>  {
>  	access_rw(vcpu, p, r);
>  	if (p->is_write)
> -		vcpu_set_flag(vcpu, DEBUG_DIRTY);
>  
>  	kvm_debug_set_guest_ownership(vcpu);
>  	return true;

Sorry for stating the same thing again, but this is a repeat of [1].
Should I add this on top?

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 55c49150fac40..92e2c4c12f0dc 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -619,8 +619,6 @@ static bool trap_debug_regs(struct kvm_vcpu *vcpu,
 			    const struct sys_reg_desc *r)
 {
 	access_rw(vcpu, p, r);
-	if (p->is_write)
-
 	kvm_debug_set_guest_ownership(vcpu);
 	return true;
 }

	M.

[1] https://lore.kernel.org/r/Zy_0T00hOjVfmKTz@linux.dev

-- 
Without deviation from the norm, progress is not possible.