linux-kernel - Re: [PATCH 10/22] kvm: mmu: Add TDP MMU PF handler

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Message-ID: <CANgfPd9LmFfZisR3oXKZ7xSeTRi7GyDr+VWf0dE9U4y6_icmTA@mail.gmail.com>
Date:   Tue, 6 Oct 2020 15:35:08 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 10/22] kvm: mmu: Add TDP MMU PF handler

On Wed, Sep 30, 2020 at 10:38 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 30/09/20 18:37, Sean Christopherson wrote:
> >> +    ret = page_fault_handle_target_level(vcpu, write, map_writable,
> >> +                                         as_id, &iter, pfn, prefault);
> >> +
> >> +    /* If emulating, flush this vcpu's TLB. */
> > Why?  It's obvious _what_ the code is doing, the comment should explain _why_.
> >
> >> +    if (ret == RET_PF_EMULATE)
> >> +            kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
> >> +
> >> +    return ret;
> >> +}
>
> In particular it seems to be only needed in this case...
>
> +       /*
> +        * If the page fault was caused by a write but the page is write
> +        * protected, emulation is needed. If the emulation was skipped,
> +        * the vCPU would have the same fault again.
> +        */
> +       if ((make_spte_ret & SET_SPTE_WRITE_PROTECTED_PT) && write)
> +               ret = RET_PF_EMULATE;
> +
>
> ... corresponding to this code in mmu.c
>
>         if (set_spte_ret & SET_SPTE_WRITE_PROTECTED_PT) {
>                 if (write_fault)
>                         ret = RET_PF_EMULATE;
>                 kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
>         }
>
> So it should indeed be better to make the code in
> page_fault_handle_target_level look the same as mmu/mmu.c.

That's an excellent point. I've made an effort to make them more
similar. I think this difference arose from the synchronization
changes I was working back from, but this will be much more elegant in
either case.

>
> Paolo
>