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Re: [PATCH 3/4] x86/altp2m: p2m_altp2m_get_or_propagate() should honor present page order
- To: Andrew Cooper <amc96@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 2 Dec 2021 10:55:18 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Tamas K Lengyel <tamas@xxxxxxxxxxxxx>, Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>, Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 02 Dec 2021 09:55:42 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 01.12.2021 13:44, Andrew Cooper wrote:
> On 01/12/2021 10:54, Jan Beulich wrote:
>> @@ -2237,11 +2243,11 @@ bool p2m_altp2m_get_or_propagate(struct
>> * to the start of the superpage. NB that we repupose `amfn`
>> * here.
>> */
>> - mask = ~((1UL << page_order) - 1);
>> + mask = ~((1UL << cur_order) - 1);
>> amfn = _mfn(mfn_x(*mfn) & mask);
>> gfn = _gfn(gfn_l & mask);
>>
>> - rc = p2m_set_entry(ap2m, gfn, amfn, page_order, *p2mt, *p2ma);
>> + rc = p2m_set_entry(ap2m, gfn, amfn, cur_order, *p2mt, *p2ma);
>> p2m_unlock(ap2m);
>
> While I agree with the problem you've identified, this function has some
> very broken return semantics.
>
> Logically, it is taking some hostp2m properties for gfn, and replacing
> them with ap2m properties for the same gfn.
>
>
> It cannot be correct to only update the caller state on the error
> paths. At a minimum, the
>
> if ( paged )
> p2m_mem_paging_populate(currd, _gfn(gfn));
>
> path in the success case is wrong when we've adjusted gfn down.
I wonder which of the exit paths you consider to be "error" ones. The
first one returning "false" pretty clearly isn't, for example. And the
path returning "true" is after a p2m_set_entry(), which means (if that
one succeeded) that caller values are now in sync with the P2M and
hence doen't need updating afaics.
And anyway - how does what you say relate to the patch at hand? I don't
think you mean to request that I fix further problems elsewhere, right
here?
Jan
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