linux-kernel - Re: [RFC PATCH 1/3] kvm: x86: Rename KVM_DEBUGREG_RELOAD to KVM_DEBUGREG_NEED

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Date:   Fri, 24 Apr 2020 13:29:22 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Nadav Amit <namit@...technion.ac.il>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] kvm: x86: Rename KVM_DEBUGREG_RELOAD to
 KVM_DEBUGREG_NEED_RELOAD

On Fri, Apr 24, 2020 at 04:21:03PM -0400, Peter Xu wrote:
> On Thu, Apr 23, 2020 at 12:09:42PM -0700, Sean Christopherson wrote:
> > On Thu, Apr 16, 2020 at 06:15:07PM +0800, Xiaoyao Li wrote:
> > > To make it more clear that the flag means DRn (except DR7) need to be
> > > reloaded before vm entry.
> > > 
> > > Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> > > ---
> > >  arch/x86/include/asm/kvm_host.h | 2 +-
> > >  arch/x86/kvm/x86.c              | 6 +++---
> > >  2 files changed, 4 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > index c7da23aed79a..f465c76e6e5a 100644
> > > --- a/arch/x86/include/asm/kvm_host.h
> > > +++ b/arch/x86/include/asm/kvm_host.h
> > > @@ -511,7 +511,7 @@ struct kvm_pmu_ops;
> > >  enum {
> > >  	KVM_DEBUGREG_BP_ENABLED = 1,
> > >  	KVM_DEBUGREG_WONT_EXIT = 2,
> > > -	KVM_DEBUGREG_RELOAD = 4,
> > > +	KVM_DEBUGREG_NEED_RELOAD = 4,
> > 
> > My vote would be for KVM_DEBUGREG_DIRTY  Any bit that is set switch_db_regs
> > triggers a reload, whereas I would expect a RELOAD flag to be set _every_
> > time a load is needed and thus be the only bit that's checked
> 
> But then shouldn't DIRTY be set as long as KVM_DEBUGREG_BP_ENABLED is set every
> time before vmenter?  Then it'll somehow go back to switch_db_regs, iiuc...
> 
> IIUC RELOAD actually wants to say "reload only for this iteration", that's why
> it's cleared after each reload.  So maybe...  RELOAD_ONCE?

Or FORCE_LOAD, or FORCE_RELOAD?  Those crossed my mind as well.

> (Btw, do we have debug regs tests somewhere no matter inside guest or with
>  KVM_SET_GUEST_DEBUG?)

I don't think so?