linux-kernel - Re: [PATCH 2/8] KVM: pfncache: add a mark-dirty helper

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Message-ID: <87b3f6713a7c6aa57adc89b6c47be3e1511f66ca.camel@infradead.org>
Date:   Thu, 14 Sep 2023 11:21:48 +0200
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paul Durrant <paul@....org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Paul Durrant <pdurrant@...zon.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH 2/8] KVM: pfncache: add a mark-dirty helper

On Thu, 2023-09-14 at 08:49 +0000, Paul Durrant wrote:
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -430,14 +430,13 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
>                 smp_wmb();
>         }
>  
> -       if (user_len2)
> +       if (user_len2) {
> +               kvm_gpc_mark_dirty(gpc2);
>                 read_unlock(&gpc2->lock);
> +       }
>  
> +       kvm_gpc_mark_dirty(gpc1);
>         read_unlock_irqrestore(&gpc1->lock, flags);
> -
> -       mark_page_dirty_in_slot(v->kvm, gpc1->memslot, gpc1->gpa >> PAGE_SHIFT);
> -       if (user_len2)
> -               mark_page_dirty_in_slot(v->kvm, gpc2->memslot, gpc2->gpa >> PAGE_SHIFT);
>  }
>  
>  void kvm_xen_update_runstate(struct kvm_vcpu *v, int state)

ISTR there was a reason why the mark_page_dirty_in_slot() was called
*after* unlocking. Although now I say it, that seems wrong... is that
because the spinlock is only protecting the uHVA→kHVA mapping, while
the memslot/gpa are going to remain valid even after unlock, because
those are protected by sRCU?

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