Re: [Xen-devel] [PATCH 17/17] xen/arm64: Zero BSS after the MMU and D-cache is turned on

From: Julien Grall <julien.grall@arm.com>
To: Stefano Stabellini <sstabellini@kernel.org>
Cc: xen-devel@lists.xenproject.org, Oleksandr_Tyshchenko@epam.com,
	andrii_anisov@epam.com, andre.przywara@arm.com
Subject: Re: [Xen-devel] [PATCH 17/17] xen/arm64: Zero BSS after the MMU and D-cache is turned on
Date: Thu, 27 Jun 2019 12:04:20 +0100	[thread overview]
Message-ID: <97e51782-85f8-8381-4244-cd8e2e0165e2@arm.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1906261407150.5851@sstabellini-ThinkPad-T480s>

Hi Stefano,

On 26/06/2019 22:08, Stefano Stabellini wrote:
> On Wed, 26 Jun 2019, Julien Grall wrote:
>> Hi Stefano,
>>
>> On 6/26/19 8:29 PM, Stefano Stabellini wrote:
>>> On Mon, 10 Jun 2019, Julien Grall wrote:
>>>> At the moment BSS is zeroed before the MMU and D-Cache is turned on.
>>>> In other words, the cache will be bypassed when zeroing the BSS section.
>>>>
>>>> Per the Image protocol [1], the state of the cache for BSS region is not
>>>> known because it is not part of the "loaded kernel image".
>>>>
>>>> This means that the cache will need to be invalidated twice for the BSS
>>>> region:
>>>>       1) Before zeroing to remove any dirty cache line. Otherwise they may
>>>>       get evicted while zeroing and therefore overriding the value.
>>>>       2) After zeroing to remove any cache line that may have been
>>>>       speculated. Otherwise when turning on MMU and D-Cache, the CPU may
>>>>       see old values.
>>>>
>>>> However, the only reason to have the BSS zeroed early is because the
>>>> boot page tables are part of BSS. To avoid the two cache invalidations,
>>>> it is possible to move the page tables in the section .data.page_aligned.
>>>
>>> I am not following the last part. How is moving the boot page tables to
>>> .data.page_aligned solving the problem? Do we need to zero
>>> .data.page_aligned early or can we skip it because it is guaranteed to
>>> already be zero?
>>
>> Global variables are initialized to zero by default regardless the section
>> they reside. Usually they are stored in BSS because it saves space in the
>> binary.
>>
>> With the Image protocol, BSS is not initialized by the bootloader so we have
>> to do ourself.
>>
>> The section .data.page_aligned is always part of the binary. So the compiler
>> will write zero in the binary for any global variables part of that section
>> and therefore the corresponding memory will be zeroed when loading the binary.
>>
>> If it makes sense, I can try to incorporate that in the commit message.
> 
> So basically it is really only BSS the problem. All right, looks good to
> me.

Yes that's correct.

> 
> Acked-by: Stefano Stabellini <sstabellini@kernel.org>

Thank you!

Cheers,

-- 
Julien Grall

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