linux-kernel - Re: [PATCH 1/2] xen: fix setting of max_pfn in shared

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Date:   Wed, 16 Jun 2021 12:37:35 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, stable@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH 1/2] xen: fix setting of max_pfn in shared_info

On 16.06.21 11:52, Jan Beulich wrote:
> On 16.06.2021 09:30, Juergen Gross wrote:
>> Xen PV guests are specifying the highest used PFN via the max_pfn
>> field in shared_info. This value is used by the Xen tools when saving
>> or migrating the guest.
>>
>> Unfortunately this field is misnamed, as in reality it is specifying
>> the number of pages (including any memory holes) of the guest, so it
>> is the highest used PFN + 1. Renaming isn't possible, as this is a
>> public Xen hypervisor interface which needs to be kept stable.
>>
>> The kernel will set the value correctly initially at boot time, but
>> when adding more pages (e.g. due to memory hotplug or ballooning) a
>> real PFN number is stored in max_pfn. This is done when expanding the
>> p2m array, and the PFN stored there is even possibly wrong, as it
>> should be the last possible PFN of the just added P2M frame, and not
>> one which led to the P2M expansion.
>>
>> Fix that by setting shared_info->max_pfn to the last possible PFN + 1.
>>
>> Fixes: 98dd166ea3a3c3 ("x86/xen/p2m: hint at the last populated P2M entry")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Juergen Gross <jgross@...e.com>
> 
> The code change is fine, so
> Reviewed-by: Jan Beulich <jbeulich@...e.com>
> 
> But I think even before the rename you would want to clarify the comment
> next to the variable's definition, to make clear what it really holds.

It already says: "Number of valid entries in the p2m table(s) ..."
What do you think is unclear about that? Or do you mean another
variable?


Juergen

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