Re: [Xen-devel] [PATCH v5 05/12] x86: provide executable fixmap facility

From: Jan Beulich <jbeulich@suse.com>
To: Wei Liu <wl@xen.org>
Cc: "Wei Liu" <liuwe@microsoft.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Paul Durrant" <pdurrant@amazon.com>,
	"Michael Kelley" <mikelley@microsoft.com>,
	"Ross Lagerwall" <ross.lagerwall@citrix.com>,
	"Xen Development List" <xen-devel@lists.xenproject.org>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v5 05/12] x86: provide executable fixmap facility
Date: Fri, 31 Jan 2020 14:19:16 +0100	[thread overview]
Message-ID: <debd27c2-e2ba-a887-01f7-ec1e0455fc2a@suse.com> (raw)
In-Reply-To: <20200131131215.bc7daxfbasudiwbi@debian>

On 31.01.2020 14:12, Wei Liu wrote:
> On Thu, Jan 30, 2020 at 12:30:47AM +0000, Wei Liu wrote:
>> On Wed, Jan 29, 2020 at 08:20:27PM +0000, Wei Liu wrote:
>>>  
>>> +void __set_fixmap_x(
>>> +    enum fixed_addresses_x idx, unsigned long mfn, unsigned long flags)
>>> +{
>>> +    BUG_ON(idx >= __end_of_fixed_addresses_x || idx <= FIX_X_RESERVED);
>>> +    map_pages_to_xen(__fix_x_to_virt(idx), _mfn(mfn), 1, flags);
>>> +
>>> +    /* Generate a symbol to be used in linker script */
>>> +    asm ( ".equ FIXADDR_X_SIZE, %c0; .global FIXADDR_X_SIZE"
>>> +          :: "i" (__end_of_fixed_addresses_x << PAGE_SHIFT) );
>>
>> The (__end << SHIFT) part can be replaced with FIXADDR_X_SIZE (the macro
>> defined in fixmap.h) directly.
> 
> I also discover that this snippet to generate symbol should be moved
> else where because if Hyper-V support is compiled out, this function has
> no user. That causes it to be DCE'd. FIXADDR_X_SIZE would be gone and
> linking would fail.
> 
> I have moved this to arch_init_memory. Its storage will be reclaimed
> during runtime, but this symbol is not used anywhere else in code, so we
> should be fine.

Storage? This is a constant, i.e. just a symbol without any
storage. Therefore an __init function is a very good place to
put it. It could also live outside of any function, if only
file scope asm()-s finally permitted [certain] inputs.

Jan

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