RE: [PATCH v2 2/2] x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()

["Durrant, Paul" <pdurrant@amazon.co.uk>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@suse.com>
> Sent: 31 July 2020 14:41
> To: paul@xen.org
> Cc: xen-devel@lists.xenproject.org; Durrant, Paul <pdurrant@amazon.co.uk>; 'Andrew Cooper'
> <andrew.cooper3@citrix.com>; 'Wei Liu' <wl@xen.org>; 'Roger Pau Monné' <roger.pau@citrix.com>
> Subject: RE: [EXTERNAL] [PATCH v2 2/2] x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()
> 
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> On 31.07.2020 15:07, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <jbeulich@suse.com>
> >> Sent: 31 July 2020 13:58
> >> To: Paul Durrant <paul@xen.org>
> >> Cc: xen-devel@lists.xenproject.org; Paul Durrant <pdurrant@amazon.com>; Andrew Cooper
> >> <andrew.cooper3@citrix.com>; Wei Liu <wl@xen.org>; Roger Pau Monné <roger.pau@citrix.com>
> >> Subject: Re: [PATCH v2 2/2] x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()
> >>
> >> On 31.07.2020 14:39, Paul Durrant wrote:
> >>> From: Paul Durrant <pdurrant@amazon.com>
> >>>
> >>> Re-factor the code to take advantage of the fact that the APIC access page is
> >>> a 'special' page.
> >>
> >> Hmm, that's going quite as far as I was thinking to go: In
> >> particular, you leave in place the set_mmio_p2m_entry() use
> >> in vmx_alloc_vlapic_mapping(). With that replaced, the
> >> re-ordering in epte_get_entry_emt() that you do shouldn't
> >> be necessary; you'd simple drop the checking of the
> >> specific MFN.
> >
> > Ok, it still needs to go in the p2m though so are you suggesting
> > just calling p2m_set_entry() directly?
> 
> Yes, if this works. The main question really is whether there are
> any hidden assumptions elsewhere that this page gets mapped as an
> MMIO one.
> 
> >>> --- a/xen/arch/x86/hvm/mtrr.c
> >>> +++ b/xen/arch/x86/hvm/mtrr.c
> >>> @@ -814,29 +814,22 @@ int epte_get_entry_emt(struct domain *d, unsigned long gfn, mfn_t mfn,
> >>>          return -1;
> >>>      }
> >>>
> >>> -    if ( direct_mmio )
> >>> -    {
> >>> -        if ( (mfn_x(mfn) ^ mfn_x(d->arch.hvm.vmx.apic_access_mfn)) >> order )
> >>> -            return MTRR_TYPE_UNCACHABLE;
> >>> -        if ( order )
> >>> -            return -1;
> >>
> >> ... this part of the logic wants retaining, I think, i.e.
> >> reporting back to the guest that the mapping needs splitting.
> >> I'm afraid I have to withdraw my R-b on patch 1 for this
> >> reason, as the check needs to be added there already.
> >
> > To be clear... You mean I need the:
> >
> > if ( order )
> >   return -1;
> >
> > there?
> 
> Not just - first of all you need to check whether the requested
> range overlaps a special page.

Ok. I'll add that.

  Paul


> 
> Jan