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Subject: Re: [PATCH 2/2] xen: rename wrong named pfn related variables
To:     Jan Beulich <jbeulich@suse.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, x86@kernel.org
References: <20210616073007.5215-1-jgross@suse.com>
 <20210616073007.5215-3-jgross@suse.com>
 <8dbeb9ea-56c9-de30-4d5f-fc9c0ced6ac4@suse.com>
 <79434ec4-4543-97ad-b010-3f2c1b6a55ad@suse.com>
 <b9c64bcd-4192-0075-ddf5-711e84301063@suse.com>
 <94d629fd-27e2-f2be-ed26-c3e04e95c5b4@suse.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <4a5d8b43-7ffa-cbcd-943c-3c459d56e55e@suse.com>
Date:   Mon, 16 Aug 2021 07:25:24 +0200
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From: Juergen Gross <jgross@suse.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 Stefano Stabellini <sstabellini@kernel.org>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
 xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, x86@kernel.org
Message-ID: <4a5d8b43-7ffa-cbcd-943c-3c459d56e55e@suse.com>
Subject: Re: [PATCH 2/2] xen: rename wrong named pfn related variables
References: <20210616073007.5215-1-jgross@suse.com>
 <20210616073007.5215-3-jgross@suse.com>
 <8dbeb9ea-56c9-de30-4d5f-fc9c0ced6ac4@suse.com>
 <79434ec4-4543-97ad-b010-3f2c1b6a55ad@suse.com>
 <b9c64bcd-4192-0075-ddf5-711e84301063@suse.com>
 <94d629fd-27e2-f2be-ed26-c3e04e95c5b4@suse.com>
In-Reply-To: <94d629fd-27e2-f2be-ed26-c3e04e95c5b4@suse.com>

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On 03.08.21 12:42, Jan Beulich wrote:
> On 30.07.2021 11:00, Juergen Gross wrote:
>> On 16.06.21 12:43, Juergen Gross wrote:
>>> On 16.06.21 11:56, Jan Beulich wrote:
>>>> On 16.06.2021 09:30, Juergen Gross wrote:
>>>>> --- a/arch/x86/xen/p2m.c
>>>>> +++ b/arch/x86/xen/p2m.c
>>>>> @@ -95,8 +95,8 @@ unsigned long *xen_p2m_addr __read_mostly;
>>>>>  =C2=A0 EXPORT_SYMBOL_GPL(xen_p2m_addr);
>>>>>  =C2=A0 unsigned long xen_p2m_size __read_mostly;
>>>>>  =C2=A0 EXPORT_SYMBOL_GPL(xen_p2m_size);
>>>>> -unsigned long xen_max_p2m_pfn __read_mostly;
>>>>> -EXPORT_SYMBOL_GPL(xen_max_p2m_pfn);
>>>>> +unsigned long xen_p2m_max_size __read_mostly;
>>>>> +EXPORT_SYMBOL_GPL(xen_p2m_max_size);
>>>>
>>>> Instead of renaming the exported variable (which will break consumer=
s
>>>> anyway), how about dropping the apparently unneeded export at this
>>>> occasion?
>>>
>>> Why do you think it isn't needed? It is being referenced via the inli=
ne
>>> function __pfn_to_mfn() in arch/x86/include/asm/xen/page.h. And
>>> __pfn_to_mfn() is used via lots of other inline functions and macros.=

>>>
>>>> Further it looks to me as if xen_p2m_size and this variable
>>>> were actually always kept in sync, so I'd like to put up the questio=
n
>>>> of dropping one of the two.
>>>
>>> Hmm, should be possible, yes.
>>
>> Looking into this it seems this is not possible.
>>
>> xen_p2m_size always holds the number of p2m entries in the p2m table,
>> including invalid ones at the end. xen_p2m_pfn_limit however contains
>> the (rounded up) index after the last valid p2m entry.
>=20
> I'm afraid I can't follow:
>=20
> xen_build_dynamic_phys_to_machine() sets xen_p2m_size and then syncs
> its value to what so far has been xen_max_p2m_pfn.
>=20
> xen_vmalloc_p2m_tree() sets xen_max_p2m_pfn and then syncs its value
> to xen_p2m_size.
>=20
> I therefore can't see how the two values would hold different values,
> except for the brief periods between updating one and then the other.

The brief period in xen_vmalloc_p2m_tree() is the problematic one. The
different values are especially important for the calls of
__pfn_to_mfn() during xen_rebuild_p2m_list().


Juergen

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