linux-kernel - Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update

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Message-ID: <2f80a561-a338-cdb3-e641-d8803b859ce0@redhat.com>
Date:   Wed, 28 Sep 2022 19:29:22 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update

On 9/9/22 12:45, Emanuele Giuseppe Esposito wrote:
> @@ -1782,7 +1782,8 @@ static void kvm_update_flags_memslot(struct kvm *kvm,
>   
>   /*
>    * Takes kvm->slots_arch_lock, and releases it only if
> - * invalid_slot allocation or kvm_prepare_memory_region failed.
> + * invalid_slot allocation, kvm_prepare_memory_region failed
> + * or batch->is_move_delete is true.
>    */

This _is_ getting out of hand, though. :)  It is not clear to me why you 
need to do multiple swaps.  If you did a single swap, you could do all 
the allocations of invalid slots in a separate loop, called with 
slots_arch_lock taken and not released until the final swap.  In other 
words, something like

	mutex_lock(&kvm->slots_arch_lock);
	r = create_invalid_slots();
	if (r < 0)
		return r;

	replace_old_slots();

	// also handles abort on failure
	prepare_memory_regions();
	if (r < 0)
		return r;
	swap();
	finish_memslots();

where each function is little more than a loop over the corresponding 
function called by KVM_SET_MEMORY_REGION.

Paolo