Re: [Xen-devel] [PATCH 4/8] xen: Use the correctly the Xen memory terminologies

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/29/2015 07:25 AM, Julien Grall wrote:
> Hi Boris,
>
> On 28/07/15 20:12, Boris Ostrovsky wrote:
> > On 07/28/2015 11:02 AM, Julien Grall wrote:
> > > Based on include/xen/mm.h [1], Linux is mistakenly using MFN when GFN
> > > is meant, I suspect this is because the first support for Xen was for
> > > PV. This brough some misimplementation of helpers on ARM and make the
> > > developper confused the expected behavior.
> > >
> > > For instance, with pfn_to_mfn, we expect to get an MFN based on the name.
> > > Although, if we look at the implementation on x86, it's returning a GFN.
> > >
> > > For clarity and avoid new confusion, replace any reference of mfn into
> > > gnf in any helpers used by PV drivers.
> >
> >
> >
> > > @@ -730,7 +730,7 @@ static void xen_do_pin(unsigned level, unsigned
> > > long pfn)
> > >        struct mmuext_op op;
> > >
> > >        op.cmd = level;
> > > -    op.arg1.mfn = pfn_to_mfn(pfn);
> > > +    op.arg1.mfn = pfn_to_gfn(pfn);
> >
> > This looks slightly odd. It is correct but given that purpose of this
> > series is to make things more clear perhaps we can add another union
> > member (gfn) to mmuext_op.arg1?
> >
> > (Of course, the hypervisor will continue referring to mfn which could
> > still be confusing)
> This operation is only used for PV guests, right?
>
> IHMO re-introducing pfn_to_mfn for PV-guests only (i.e with a BUG_ON to
> ensure no usage for auto-translated guest) would be the best solution.
> It would avoid to have different name than the hypersivor one in the
> hypercall interface. It will also make clear that virt_to_machine & co
> is only PV specific.
>
> I though doing this but I preferred to defer it to x86 expert as my
> knowledge for x86 Xen is very limited. I don't know where it's more
> suitable to use MFN or GFN. I guess this file (mmu.c) is mostly PV specific?
>
> Would something like below fine for you?
>
> static inline unsigned long pfn_to_mfn(unsigned long pfn)
> {
>         unsigned long mfn;
>
>         BUG_ON(xen_feature(XENFEAT_auto_translated_physmap));
>
>         mfn = __pfn_to_mfn(pfn);
>         if (mfn != INVALID_P2M_ENTRY)
>                 mfn &= ~(FOREIGN_FRAME_BIT | IDENTITY_FRAME_BIT);
>
>         return mfn;
> }
>
> static inline unsigned long pfn_to_gfn(unsigned long pfn)
> {
>         if (xen_feature(XENFEAT_autotranslated_physmap))
>                 return pfn;
>         else
>                 return pfn_to_mfn(pfn);
> }

But you'd still say 'op.arg1.mfn = pfn_to_gfn(pfn);' in xen_do_pin() 
i.e. assign GFN to MFN, right? That's what I was referring to.
(In general, I am not sure a guest should ever use 'mfn' as it is purely 
a hypervisor construct. Including p2m, which I think should really be 
p2g as this is what we use to figure out what to stick into page tables)
-boris


> Similar splitting would be done for gfn_to_pfn and mfn_to_pfn.
>
> Regards,

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