Re: [Xen-devel] [PATCH v5] x86/altp2m: Aggregate get entry and populate into common funcs

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On 16.04.2019 18:07, George Dunlap wrote:
> On 4/16/19 3:19 PM, Tamas K Lengyel wrote:
>> On Tue, Apr 16, 2019 at 8:02 AM George Dunlap <george.dunlap@xxxxxxxxxx> 
>> wrote:
>>>
>>> On 4/16/19 2:44 PM, Tamas K Lengyel wrote:
>>>> On Tue, Apr 16, 2019 at 2:45 AM Alexandru Stefan ISAILA
>>>> <aisaila@xxxxxxxxxxxxxxx> wrote:
>>>>>
>>>>> The code for getting the entry and then populating was repeated in
>>>>> p2m_change_altp2m_gfn() and in p2m_set_altp2m_mem_access().
>>>>>
>>>>> The code is now in one place with a bool param that lets the caller choose
>>>>> if it populates after get_entry().
>>>>>
>>>>> If remapping is being done then both the old and new gfn's should be
>>>>> unshared in the hostp2m for keeping things consistent. The page type
>>>>> of old_gfn was already checked whether it's p2m_ram_rw and bail if it
>>>>> wasn't so functionality-wise this just simplifies things as a user
>>>>> doesn't have to request unsharing manually before remapping.
>>>>> Now, if the new_gfn is invalid it shouldn't query the hostp2m as
>>>>> that is effectively a request to remove the entry from the altp2m.
>>>>> But provided that scenario is used only when removing entries that
>>>>> were previously remapped/copied to the altp2m, those entries already
>>>>> went through P2M_ALLOC | P2M_UNSHARE before, so it won't have an
>>>>> affect so the core function get_altp2m_entry() is calling
>>>>> __get_gfn_type_access() with P2M_ALLOC | P2M_UNSHARE.
>>>>>
>>>>> altp2m_get_entry_direct() is also called in p2m_set_suppress_ve()
>>>>> because on a new altp2m view the function will fail with invalid mfn if
>>>>> p2m->set_entry() was not called before.
>>>>>
>>>>> Signed-off-by: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
>>>>> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
>>>>> Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
>>>>>
>>>>> ---
>>>>> Changes since V4:
>>>>>          - Add altp2m to patch name
>>>>>          - Change func name from get_altp2m_entry() to
>>>>> altp2m_get_entry().
>>>>> ---
>>>>>   xen/arch/x86/mm/mem_access.c | 30 ++-----------
>>>>>   xen/arch/x86/mm/p2m.c        | 84 ++++++++++++++++++++----------------
>>>>>   xen/include/asm-x86/p2m.h    | 17 ++++++++
>>>>>   3 files changed, 66 insertions(+), 65 deletions(-)
>>>>>
>>>>> diff --git a/xen/arch/x86/mm/mem_access.c b/xen/arch/x86/mm/mem_access.c
>>>>> index a144bb0ce4..ddfe0169c0 100644
>>>>> --- a/xen/arch/x86/mm/mem_access.c
>>>>> +++ b/xen/arch/x86/mm/mem_access.c
>>>>> @@ -262,35 +262,11 @@ int p2m_set_altp2m_mem_access(struct domain *d, 
>>>>> struct p2m_domain *hp2m,
>>>>>       mfn_t mfn;
>>>>>       p2m_type_t t;
>>>>>       p2m_access_t old_a;
>>>>> -    unsigned int page_order;
>>>>> -    unsigned long gfn_l = gfn_x(gfn);
>>>>>       int rc;
>>>>>
>>>>> -    mfn = ap2m->get_entry(ap2m, gfn, &t, &old_a, 0, NULL, NULL);
>>>>> -
>>>>> -    /* Check host p2m if no valid entry in alternate */
>>>>> -    if ( !mfn_valid(mfn) )
>>>>> -    {
>>>>> -
>>>>> -        mfn = __get_gfn_type_access(hp2m, gfn_l, &t, &old_a,
>>>>> -                                    P2M_ALLOC | P2M_UNSHARE, 
>>>>> &page_order, 0);
>>>>> -
>>>>> -        rc = -ESRCH;
>>>>> -        if ( !mfn_valid(mfn) || t != p2m_ram_rw )
>>>>> -            return rc;
>>>>> -
>>>>> -        /* If this is a superpage, copy that first */
>>>>> -        if ( page_order != PAGE_ORDER_4K )
>>>>> -        {
>>>>> -            unsigned long mask = ~((1UL << page_order) - 1);
>>>>> -            gfn_t gfn2 = _gfn(gfn_l & mask);
>>>>> -            mfn_t mfn2 = _mfn(mfn_x(mfn) & mask);
>>>>> -
>>>>> -            rc = ap2m->set_entry(ap2m, gfn2, mfn2, page_order, t, old_a, 
>>>>> 1);
>>>>> -            if ( rc )
>>>>> -                return rc;
>>>>> -        }
>>>>> -    }
>>>>> +    rc = altp2m_get_entry_prepopulate(ap2m, gfn, &mfn, &t, &old_a);
>>>>> +    if ( rc )
>>>>> +        return rc;
>>>>>
>>>>>       /*
>>>>>        * Inherit the old suppress #VE bit value if it is already set, or 
>>>>> set it
>>>>> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
>>>>> index 9e81a30cc4..7bedfd593b 100644
>>>>> --- a/xen/arch/x86/mm/p2m.c
>>>>> +++ b/xen/arch/x86/mm/p2m.c
>>>>
>>>> Wouldn't it make more sense to start adding new altp2m functions to
>>>> mm/altp2m.c instead? Probably the altp2m functions from mm/p2m.c could
>>>> also be relocated there at some point in the future.
>>>>
>>>>> @@ -478,6 +478,43 @@ void p2m_unlock_and_tlb_flush(struct p2m_domain *p2m)
>>>>>           mm_write_unlock(&p2m->lock);
>>>>>   }
>>>>>
>>>>> +int altp2m_get_entry(struct p2m_domain *ap2m,
>>>>> +                            gfn_t gfn, mfn_t *mfn, p2m_type_t *t,
>>>>> +                            p2m_access_t *a, bool prepopulate)
>>>>> +{
>>>>> +    *mfn = ap2m->get_entry(ap2m, gfn, t, a, 0, NULL, NULL);
>>>>> +
>>>>> +    /* Check host p2m if no valid entry in alternate */
>>>>> +    if ( !mfn_valid(*mfn) && !p2m_is_hostp2m(ap2m) )
>>>>> +    {
>>>>> +        struct p2m_domain *hp2m = p2m_get_hostp2m(ap2m->domain);
>>>>> +        unsigned int page_order;
>>>>> +        int rc;
>>>>> +
>>>>> +        *mfn = __get_gfn_type_access(hp2m, gfn_x(gfn), t, a,
>>>>> +                                     P2M_ALLOC | P2M_UNSHARE, 
>>>>> &page_order, 0);
>>>>
>>>> So despite the name being altp2m_get_entry you now return an entry
>>>> from the hostp2m, even if prepopulate is false. If the caller knows it
>>>> doesn't want that entry to be copied into the altp2m, why not have it
>>>> call __get_gfn_type_access itself for the hostp2m? IMHO this is just
>>>> confusing and doesn't help readability of the altp2m code.
>>>
>>> You return the ap2m entry if it's present, or the hp2m entry if it's
>>> not.  It's not a lot of duplication, but it makes the logic cleaner I
>>> think; why not deduplicate it?
>>
>> I have no problem with making the code more streamlined. The problem I
>> have is that the function's name doesn't suggest it would get you
>> anything but the entry from the specified altp2m. So you could be
>> reading the code assuming you are dealing with an entry from that
>> specified table when in fact you are not. That is not an expected
>> behavior based on just the name of the function. This is going to make
>> reading the altp2m code that much harder in the future.
> 
> Right -- I wasn't a huge fan of 'direct' either; it didn't really convey
> to me 100% what the function did.  My PoC had "seethrough", but that
> wasn't that great either.  "Peek"?  Any other suggestions?
> 
> Other options:
> 
> * If we have a single function with a #define, this might get a bit
> easier;  we could have one be AP2MGET_dont_prepopulate or something.
> 
> ( We could have the "core" function named _altp2m_get_entry, and have
> altp2m_get_entry() call with prepopulate = false, and
> altp2m_get_entry_prepopulate() call it with prepopulate = true.

This option with no defines seems to solve more of the naming problems 
but it will still introduce the spaghetti code. I vote for this one and 
if Tamas agrees I will have it this way in the next version.

Alex
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