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

[Ben Gardon]

On Wed, Sep 30, 2020 at 10:38 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> 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
>