linux-kernel - Re: [PATCH v2 06/13] KVM: arm64: Add support for KVM_MEM

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Message-ID: <aDdILHOu9g-m5hSm@google.com>
Date: Wed, 28 May 2025 10:30:20 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Yan Zhao <yan.y.zhao@...el.com>, 
	Nikita Kalyazin <kalyazin@...zon.com>, Anish Moorthy <amoorthy@...gle.com>, 
	Peter Gonda <pgonda@...gle.com>, Peter Xu <peterx@...hat.com>, 
	David Matlack <dmatlack@...gle.com>, wei.w.wang@...el.com, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v2 06/13] KVM: arm64: Add support for KVM_MEM_USERFAULT

On Wed, May 28, 2025, James Houghton wrote:
> On Wed, May 28, 2025 at 11:09 AM James Houghton <jthoughton@...gle.com> wrote:
> >
> > On Tue, May 6, 2025 at 8:06 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > On Thu, Jan 09, 2025, James Houghton wrote:
> > > > @@ -2073,6 +2080,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> > > >                                  enum kvm_mr_change change)
> > > >  {
> > > >       bool log_dirty_pages = new && new->flags & KVM_MEM_LOG_DIRTY_PAGES;
> > > > +     u32 new_flags = new ? new->flags : 0;
> > > > +     u32 changed_flags = (new_flags) ^ (old ? old->flags : 0);
> > >
> > > This is a bit hard to read, and there's only one use of log_dirty_pages.  With
> > > zapping handled in common KVM, just do:
> >
> > Thanks, Sean. Yeah what you have below looks a lot better, thanks for
> > applying it for me. I'll post a new version soon. One note below.
> >
> > >
> > > @@ -2127,14 +2131,19 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> > >                                    const struct kvm_memory_slot *new,
> > >                                    enum kvm_mr_change change)
> > >  {
> > > -       bool log_dirty_pages = new && new->flags & KVM_MEM_LOG_DIRTY_PAGES;
> > > +       u32 old_flags = old ? old->flags : 0;
> > > +       u32 new_flags = new ? new->flags : 0;
> > > +
> > > +       /* Nothing to do if not toggling dirty logging. */
> > > +       if (!((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES))
> > > +               return;
> >
> > This is my bug, not yours, but I think this condition must also check
> > that `change == KVM_MR_FLAGS_ONLY` for it to be correct. This, for
> > example, will break the case where we are deleting a memslot that
> > still has KVM_MEM_LOG_DIRTY_PAGES enabled. Will fix in the next
> > version.
> 
> Ah it wouldn't break that example, as `new` would be NULL. But I think
> it would break the case where we are moving a memslot that keeps
> `KVM_MEM_LOG_DIRTY_PAGES`.

Can you elaborate?  Maybe with the full snippet of the final code that's broken.
I'm not entirely following what's path you're referring to.