Re: [Xen-devel] [PATCH 3/4] xen/link: Fold .data.read_mostly into .data

From: Julien Grall <julien.grall@arm.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>,
	Xen-devel <xen-devel@lists.xenproject.org>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>, "Jan Beulich" <JBeulich@suse.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 3/4] xen/link: Fold .data.read_mostly into .data
Date: Thu, 20 Jun 2019 13:24:03 +0100	[thread overview]
Message-ID: <74705f1c-d0ac-27fd-3c88-a58adfcec81d@arm.com> (raw)
In-Reply-To: <5cb62752-e2a1-444e-e3e4-98b25dcfb626@citrix.com>

Hi Andrew,

On 6/19/19 10:48 PM, Andrew Cooper wrote:
> On 19/06/2019 22:28, Julien Grall wrote:
>> On 6/19/19 9:11 PM, Andrew Cooper wrote:
>>> .data.read_mostly only needs separating from adjacent data by one
>>> cache line
>>> to be effective, and placing it adjacent to .data.page_aligned
>>> fulfills this
>>> requirement.
>>>
>>> For ARM, .ex_table appears to be a vestigial remnant.  Nothing in the
>>> resulting build ever inspects or acts on the contents of the table.
>>> The arm32
>>> build does however have some assembly routines which fill .ex_table.
>>>
>>> Drop all of ARM's .ex_table infrastructure, to reduce the size of the
>>> compiled
>>> binary, and avoid giving the illusion of exception handling working.
>>
>> I am not in favor of this change. assembler.h is meant to be a
>> verbatim copy of the Linux counterpart.
> 
> What alternative do you propose then, because having infrastructure that
> gives the illusion of exception recovery working is a far worse option
> than deviating from Linux.

I learnt the hard way before that trying to adapt a Linux file to Xen 
makes very difficult to keep track what's going on.

So my preference here is to just disable the section if they exists.

>>
>> [...]
>>
>>> diff --git a/xen/arch/arm/xen.lds.S b/xen/arch/arm/xen.lds.S
>>> index 2b44e5d..3dc5117 100644
>>> --- a/xen/arch/arm/xen.lds.S
>>> +++ b/xen/arch/arm/xen.lds.S
>>> @@ -94,27 +94,13 @@ SECTIONS
>>>      _erodata = .;                /* End of read-only data */
>>>        .data : {                    /* Data */
>>> +       *(.data.read_mostly)
>>
>> Before, .data.read_mostly was SMP_CACHE_BYTES aligned. Now, it seems
>> there are no alignment.
>>
>> This may end up to have _erodata and .data.read_mostly to be part of
>> the same page. As Arm enforce read-only, you may crash on access to
>> .data.read_mostly.
>>
>> So I think you have
>> . = ALIGN(PAGE_SIZE)
>> *(.data.read_mostly)
>> .align(SMP_CACHE_BYTES).
> 
> These both need to be PAGE_SIZE, or .data.page_aligned can end up having
> problems.

Good point, I missed the .data.page_aligned.

Cheers,

-- 
Julien Grall

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