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

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Message-ID: <aBqkINKO9PUAzZeS@google.com>
Date: Tue, 6 May 2025 17:06:56 -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 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:

@@ -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;
 
        /*
         * 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;