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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v5] x86/altp2m: Aggregate get entry and populate into common funcs
On Wed, Apr 17, 2019 at 1:15 AM Alexandru Stefan ISAILA
<aisaila@xxxxxxxxxxxxxxx> wrote:
>
>
>
> 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.
>
Having altp2m_get_entry and altp2m_get_entry_prepopulate seem to be a
better name for them, as long as altp2m_get_entry doesn't return an
entry from the hostp2m if there isn't one in the altp2m, and
altp2m_get_entry_prepopulate returns an entry only if prepopulation
actually worked. In both of those cases the functions would only
return entries from the altp2m, as their name actually suggests.
Tamas
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