linux-kernel - Re: [PATCH v6 25/29] KVM: Call kvm_arch_flush_shadow_memslot() on the old slot in kvm_invalidate

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Message-ID: <Yabrr6Q9WxFb3Eec@google.com>
Date:   Wed, 1 Dec 2021 03:27:43 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Igor Mammedov <imammedo@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Atish Patra <atish.patra@....com>,
        Ben Gardon <bgardon@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 25/29] KVM: Call kvm_arch_flush_shadow_memslot() on
 the old slot in kvm_invalidate_memslot()

On Tue, Nov 30, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
> 
> kvm_invalidate_memslot() calls kvm_arch_flush_shadow_memslot() on the
> active, but KVM_MEMSLOT_INVALID slot.
> Do it on the inactive (but valid) old slot instead since arch code really
> should not get passed such invalid slot.

One other thing that's worth noting in the changelog is that "old->arch" may have
stale data.  IMO that's perfectly ok, but it's definitely a quirk.  Ideally KVM
would disallow touching "arch" for an INVALID slot, but that would require another
arch hook if kvm_prepare_memory_region() failed to refresh old->arch if necessary
before restoring it. :-/

Paolo, thoughts on this goofy case?  I don't love it, but I dislike having

	kvm_arch_flush_shadow_memslot(kvm, invalid_slot);

in the final code even more.

Reviewed-by: Sean Christopherson <seanjc@...gle.com> 

> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> ---
>  virt/kvm/kvm_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c57748ee41e8..086f18969bc3 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1632,7 +1632,7 @@ static void kvm_invalidate_memslot(struct kvm *kvm,
>  	 *	- gfn_to_hva (kvm_read_guest, gfn_to_pfn)
>  	 *	- kvm_is_visible_gfn (mmu_check_root)
>  	 */
> -	kvm_arch_flush_shadow_memslot(kvm, working_slot);
> +	kvm_arch_flush_shadow_memslot(kvm, old);
>  
>  	/* Was released by kvm_swap_active_memslots, reacquire. */
>  	mutex_lock(&kvm->slots_arch_lock);