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

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Message-ID: <CADrL8HXDDRC6Ey5HYWvtzQzjcM2RNX7c7ngGyjUsD3WiBF3VYA@mail.gmail.com>
Date: Wed, 28 May 2025 11:09:58 -0400
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...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 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.

>
>         /*
>          * At this point memslot has been committed and there is an
>          * allocated dirty_bitmap[], dirty pages will be tracked while the
>          * memory slot is write protected.
>          */
> -       if (log_dirty_pages) {
> +       if (new_flags & KVM_MEM_LOG_DIRTY_PAGES) {
>
>                 if (change == KVM_MR_DELETE)
>                         return;