linux-kernel - Re: [Patch V2 2/2] xen: before ballooning hotplugged memory, set frames to invalid

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Date:	Fri, 20 Mar 2015 15:27:54 +0100
From:	Juergen Gross <jgross@...e.com>
To:	Daniel Kiper <daniel.kiper@...cle.com>
CC:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, david.vrabel@...rix.com,
	boris.ostrovsky@...cle.com, pebolle@...cali.nl
Subject: Re: [Patch V2 2/2] xen: before ballooning hotplugged memory, set
 frames to invalid

On 03/20/2015 02:46 PM, Daniel Kiper wrote:
> On Fri, Mar 20, 2015 at 01:55:39PM +0100, Juergen Gross wrote:
>> Commit 25b884a83d487fd62c3de7ac1ab5549979188482 ("x86/xen: set
>> regions above the end of RAM as 1:1") introduced a regression.
>>
>> To be able to add memory pages which were added via memory hotplug to
>> a pv domain, the pages must be "invalid" instead of "identity" in the
>> p2m list before they can be added.
>>
>> Suggested-by: David Vrabel <david.vrabel@...rix.com>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>
> In general...
>
> Reviewed-by: Daniel Kiper <daniel.kiper@...cle.com>
>
> ... but...
>
>> ---
>>   drivers/xen/balloon.c | 13 +++++++++++++
>>   1 file changed, 13 insertions(+)
>>
>> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
>> index 0b52d92..65fedb8 100644
>> --- a/drivers/xen/balloon.c
>> +++ b/drivers/xen/balloon.c
>> @@ -229,6 +229,19 @@ static enum bp_state reserve_additional_memory(long credit)
>>   	balloon_hotplug = round_up(balloon_hotplug, PAGES_PER_SECTION);
>>   	nid = memory_add_physaddr_to_nid(hotplug_start_paddr);
>>
>> +#ifdef CONFIG_XEN_HAVE_PVMMU
>> +	if (!xen_feature(XENFEAT_auto_translated_physmap)) {
>> +		unsigned long pfn, i;
>> +
>> +		pfn = PFN_DOWN(hotplug_start_paddr);
>> +		for (i = 0; i < balloon_hotplug; i++)
>> +			if (!set_phys_to_machine(pfn + i, INVALID_P2M_ENTRY)) {
>> +				pr_warn("set_phys_to_machine() failed, no memory added\n");
>> +				return BP_ECANCELED;
>> +			}
>> +	}
>> +#endif
>
> Should not we fill everything above "maxmem" with
> INVALID_P2M_ENTRY at boot time?

In this case we shouldn't report "identity" for pfns above maxmem, too.
Otherwise we could change behaviour of the kernel regarding PCI
passthrough just by changing CONFIG_XEN_BALLOON_MEMORY_HOTPLUG_LIMIT
which doesn't sound right.


Juergen
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