Date:   Wed, 24 Nov 2021 11:33:29 +0200
From:   Oleksandr <olekstysh@...il.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     jgross@...e.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Julien Grall <julien@....org>
Subject: Re: [PATCH V2 3/4] xen/unpopulated-alloc: Add mechanism to use Xen
 resource


On 23.11.21 23:25, Stefano Stabellini wrote:

Hi Stefano

> On Tue, 23 Nov 2021, Oleksandr wrote:
>>> Actually after reading your replies and explanation I changed opinion: I
>>> think we do need the fallback because Linux cannot really assume that
>>> it is running on "new Xen" so it definitely needs to keep working if
>>> CONFIG_XEN_UNPOPULATED_ALLOC is enabled and the extended regions are not
>>> advertised.
>>>
>>> I think we'll have to roll back some of the changes introduced by
>>> 121f2faca2c0a. That's because even if CONFIG_XEN_UNPOPULATED_ALLOC is
>>> enabled we cannot know if we can use unpopulated-alloc or whether we
>>> have to use alloc_xenballooned_pages until we parse the /hypervisor node
>>> in device tree at runtime.
>> Exactly!
>>
>>
>>> In short, we cannot switch between unpopulated-alloc and
>>> alloc_xenballooned_pages at build time, we have to do it at runtime
>>> (boot time).
>> +1
>>
>>
>> I created a patch to partially revert 121f2faca2c0a "xen/balloon: rename
>> alloc/free_xenballooned_pages".
>>
>> If there is no objections I will add it to V3 (which is almost ready, except
>> the fallback bits). Could you please tell me what do you think?
>   
> It makes sense to me. You can add my Reviewed-by.

Great, thank you!


>
>   
>>  From dc79bcd425358596d95e715a8bd8b81deaaeb703 Mon Sep 17 00:00:00 2001
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>> Date: Tue, 23 Nov 2021 18:14:41 +0200
>> Subject: [PATCH] xen/balloon: Bring alloc(free)_xenballooned_pages helpers
>>   back
>>
>> This patch rolls back some of the changes introduced by commit
>> 121f2faca2c0a "xen/balloon: rename alloc/free_xenballooned_pages"
>> in order to make possible to still allocate xenballooned pages
>> if CONFIG_XEN_UNPOPULATED_ALLOC is enabled.
>>
>> On Arm the unpopulated pages will be allocated on top of extended
>> regions provided by Xen via device-tree (the subsequent patches
>> will add required bits to support unpopulated-alloc feature on Arm).
>> The problem is that extended regions feature has been introduced
>> into Xen quite recently (during 4.16 release cycle). So this
>> effectively means that Linux must only use unpopulated-alloc on Arm
>> if it is running on "new Xen" which advertises these regions.
>> But, it will only be known after parsing the "hypervisor" node
>> at boot time, so before doing that we cannot assume anything.
>>
>> In order to keep working if CONFIG_XEN_UNPOPULATED_ALLOC is enabled
>> and the extended regions are not advertised (Linux is running on
>> "old Xen", etc) we need the fallback to alloc_xenballooned_pages().
>>
>> This way we wouldn't reduce the amount of memory usable (wasting
>> RAM pages) for any of the external mappings anymore (and eliminate
>> XSA-300) with "new Xen", but would be still functional ballooning
>> out RAM pages with "old Xen".
>>
>> Also rename alloc(free)_xenballooned_pages to xen_alloc(free)_ballooned_pages.
>>
>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>> ---
>>   drivers/xen/balloon.c | 20 +++++++++-----------
>>   include/xen/balloon.h |  3 +++
>>   include/xen/xen.h     |  6 ++++++
>>   3 files changed, 18 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
>> index ba2ea11..a2c4fc49 100644
>> --- a/drivers/xen/balloon.c
>> +++ b/drivers/xen/balloon.c
>> @@ -581,7 +581,6 @@ void balloon_set_new_target(unsigned long target)
>>   }
>>   EXPORT_SYMBOL_GPL(balloon_set_new_target);
>>
>> -#ifndef CONFIG_XEN_UNPOPULATED_ALLOC
>>   static int add_ballooned_pages(unsigned int nr_pages)
>>   {
>>       enum bp_state st;
>> @@ -610,12 +609,12 @@ static int add_ballooned_pages(unsigned int nr_pages)
>>   }
>>
>>   /**
>> - * xen_alloc_unpopulated_pages - get pages that have been ballooned out
>> + * xen_alloc_ballooned_pages - get pages that have been ballooned out
>>    * @nr_pages: Number of pages to get
>>    * @pages: pages returned
>>    * @return 0 on success, error otherwise
>>    */
>> -int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
>> +int xen_alloc_ballooned_pages(unsigned int nr_pages, struct page **pages)
>>   {
>>       unsigned int pgno = 0;
>>       struct page *page;
>> @@ -652,23 +651,23 @@ int xen_alloc_unpopulated_pages(unsigned int nr_pages,
>> struct page **pages)
>>       return 0;
>>    out_undo:
>>       mutex_unlock(&balloon_mutex);
>> -    xen_free_unpopulated_pages(pgno, pages);
>> +    xen_free_ballooned_pages(pgno, pages);
>>       /*
>> -     * NB: free_xenballooned_pages will only subtract pgno pages, but since
>> +     * NB: xen_free_ballooned_pages will only subtract pgno pages, but since
>>        * target_unpopulated is incremented with nr_pages at the start we need
>>        * to remove the remaining ones also, or accounting will be screwed.
>>        */
>>       balloon_stats.target_unpopulated -= nr_pages - pgno;
>>       return ret;
>>   }
>> -EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
>> +EXPORT_SYMBOL(xen_alloc_ballooned_pages);
>>
>>   /**
>> - * xen_free_unpopulated_pages - return pages retrieved with
>> get_ballooned_pages
>> + * xen_free_ballooned_pages - return pages retrieved with get_ballooned_pages
>>    * @nr_pages: Number of pages
>>    * @pages: pages to return
>>    */
>> -void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
>> +void xen_free_ballooned_pages(unsigned int nr_pages, struct page **pages)
>>   {
>>       unsigned int i;
>>
>> @@ -687,9 +686,9 @@ void xen_free_unpopulated_pages(unsigned int nr_pages,
>> struct page **pages)
>>
>>       mutex_unlock(&balloon_mutex);
>>   }
>> -EXPORT_SYMBOL(xen_free_unpopulated_pages);
>> +EXPORT_SYMBOL(xen_free_ballooned_pages);
>>
>> -#if defined(CONFIG_XEN_PV)
>> +#if defined(CONFIG_XEN_PV) && !defined(CONFIG_XEN_UNPOPULATED_ALLOC)
>>   static void __init balloon_add_region(unsigned long start_pfn,
>>                         unsigned long pages)
>>   {
>> @@ -712,7 +711,6 @@ static void __init balloon_add_region(unsigned long
>> start_pfn,
>>       balloon_stats.total_pages += extra_pfn_end - start_pfn;
>>   }
>>   #endif
>> -#endif
>>
>>   static int __init balloon_init(void)
>>   {
>> diff --git a/include/xen/balloon.h b/include/xen/balloon.h
>> index e93d4f0..f78a6cc 100644
>> --- a/include/xen/balloon.h
>> +++ b/include/xen/balloon.h
>> @@ -26,6 +26,9 @@ extern struct balloon_stats balloon_stats;
>>
>>   void balloon_set_new_target(unsigned long target);
>>
>> +int xen_alloc_ballooned_pages(unsigned int nr_pages, struct page **pages);
>> +void xen_free_ballooned_pages(unsigned int nr_pages, struct page **pages);
>> +
>>   #ifdef CONFIG_XEN_BALLOON
>>   void xen_balloon_init(void);
>>   #else
>> diff --git a/include/xen/xen.h b/include/xen/xen.h
>> index 9f031b5..410e3e4 100644
>> --- a/include/xen/xen.h
>> +++ b/include/xen/xen.h
>> @@ -52,7 +52,13 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
>>   extern u64 xen_saved_max_mem_size;
>>   #endif
>>
>> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
>>   int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages);
>>   void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages);
>> +#else
>> +#define xen_alloc_unpopulated_pages xen_alloc_ballooned_pages
>> +#define xen_free_unpopulated_pages xen_free_ballooned_pages
>> +#include <xen/balloon.h>
>> +#endif
>>
>>   #endif    /* _XEN_XEN_H */

-- 
Regards,

Oleksandr Tyshchenko

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