From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Wei Liu <wei.liu@kernel.org>, David Hildenbrand <david@redhat.com>
Cc:     linux-hyperv@vger.kernel.org,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Michael Kelley <mikelley@microsoft.com>,
        Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC] Drivers: hv: balloon: Temporary disable the driver
 on ARM64 when PAGE_SIZE != 4k
In-Reply-To: <20220105202203.evk7uckmephnu3ev@liuwe-devbox-debian-v2>
References: <20220105165028.1343706-1-vkuznets@redhat.com>
 <20220105202203.evk7uckmephnu3ev@liuwe-devbox-debian-v2>
Date:   Thu, 06 Jan 2022 09:46:07 +0100
Message-ID: <87leztp7zk.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Wei Liu <wei.liu@kernel.org> writes:

> On Wed, Jan 05, 2022 at 05:50:28PM +0100, Vitaly Kuznetsov wrote:
>> Hyper-V ballooning and memory hotplug protocol always seems to assume
>> 4k page size so all PFNs in the structures used for communication are
>> 4k PFNs. In case a different page size is in use on the guest (e.g.
>> 64k), things go terribly wrong all over:
>> - When reporting statistics, post_status() reports them in guest pages
>> and hypervisor sees very low memory usage.
>> - When ballooning, guest reports back PFNs of the allocated pages but
>> the hypervisor treats them as 4k PFNs.
>> - When unballooning or memory hotplugging, PFNs coming from the host
>> are 4k PFNs and they may not even be 64k aligned making it difficult
>> to handle.
>> 
>> While statistics and ballooning requests would be relatively easy to
>> handle by converting between guest and hypervisor page sizes in the
>> communication structures, handling unballooning and memory hotplug
>> requests seem to be harder. In particular, when ballooning up
>> alloc_balloon_pages() shatters huge pages so unballooning request can
>> be handled for any part of it. It is not possible to shatter a 64k
>> page into 4k pages so it's unclear how to handle unballooning for a
>> sub-range if such request ever comes so we can't just report a 64k
>> page as 16 separate 4k pages.
>> 
>
> How does virtio-balloon handle it? Does its protocol handle different
> page sizes?
>

Let's ask the expert)

David,

how does virtio-balloon (and virtio-mem) deal with different page sizes
between guest and host?

>
>> Ideally, the protocol between the guest and the host should be changed
>> to allow for different guest page sizes.
>> 
>> While there's no solution for the above mentioned problems, it seems
>> we're better off without the driver in problematic cases.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  drivers/hv/Kconfig | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
>> index 0747a8f1fcee..fb353a13e5c4 100644
>> --- a/drivers/hv/Kconfig
>> +++ b/drivers/hv/Kconfig
>> @@ -25,7 +25,7 @@ config HYPERV_UTILS
>>  
>>  config HYPERV_BALLOON
>>  	tristate "Microsoft Hyper-V Balloon driver"
>> -	depends on HYPERV
>> +	depends on HYPERV && (X86 || (ARM64 && ARM64_4K_PAGES))
>>  	select PAGE_REPORTING
>>  	help
>>  	  Select this option to enable Hyper-V Balloon driver.
>> -- 
>> 2.33.1
>> 
>

-- 
Vitaly