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Re: [PATCH 1/2] x86/p2m: make p2m_get_page_from_gfn() handle grant and shared cases better
- To: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 24 Feb 2022 08:54:01 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>
- Delivery-date: Thu, 24 Feb 2022 07:54:14 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 23.02.2022 19:11, Tamas K Lengyel wrote:
>> @@ -607,6 +607,7 @@ struct page_info *p2m_get_page_from_gfn(
>>
>> /* Error path: not a suitable GFN at all */
>> if ( !p2m_is_ram(*t) && !p2m_is_paging(*t) && !p2m_is_pod(*t) &&
>> + (!p2m_is_shared(*t) || !(q & P2M_UNSHARE)) &&
>> !mem_sharing_is_fork(p2m->domain) )
>> return NULL;
>> }
>
> I don't follow what this is fixing. A shared entry would return true
> to p2m_is_ram() - p2m_ram_shared is listed under P2M_RAM_TYPES - so
> the rest of the if statement would never be checked. So if we get past
> that check we know we definitely don't have a shared entry, ie
> p2m_is_shared must be false ie the check for P2M_UNSHARE is dead code.
> Am I missing something?
No, I am. I mistakenly took p2m_is_any_ram() to include the shared case,
but p2m_is_ram() to not do so. Thanks for pointing out, and I'm actually
happy to be able to droop this hunk.
Jan
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