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);
}
Similar splitting would be done for gfn_to_pfn and mfn_to_pfn.
Regards,
--
Julien Grall
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