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From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v2 2/2] x86/desc: Build boot_{, compat_}gdt[] in C
Date: Fri, 9 Aug 2019 15:18:47 +0200
Message-ID: <ef79ee24-af84-aabd-95f5-a4363ec2a78d@suse.com> (raw)
In-Reply-To: <070c7421-04b6-23e5-88a8-7d05bb26568c@citrix.com>

On 09.08.2019 15:07, Andrew Cooper wrote:
> On 09/08/2019 13:43, Jan Beulich wrote:
>> On 09.08.2019 14:19, Andrew Cooper wrote:
>>> On 09/08/2019 11:40, Jan Beulich wrote:
>>>> --- /dev/null
>>>> +++ b/xen/arch/x86/desc.c
>>>> @@ -0,0 +1,109 @@
>>>> +
>>>> +#define SEL2GDT(sel) (((sel) >> 3) - FIRST_RESERVED_GDT_ENTRY)
>>>> +
>>>> +__section(".data.page_aligned") __aligned(PAGE_SIZE)
>>>> +seg_desc_t boot_gdt[PAGE_SIZE / sizeof(seg_desc_t)] =
>>>> +{
>>>> +    /* 0xe008 - Ring 0 code, 64bit mode */
>>>> +    [SEL2GDT(__HYPERVISOR_CS64)] = { 0x00af9b000000ffff },
>>>> +
>>>> +    /* 0xe010 - Ring 0 data */
>>>> +    [SEL2GDT(__HYPERVISOR_DS32)] = { 0x00cf93000000ffff },
>>>> +
>>>> +    /* 0xe018 - reserved */
>>>> +
>>>> +    /* 0xe023 - Ring 3 code, compatibility */
>>>> +    [SEL2GDT(FLAT_RING3_CS32)] = { 0x00cffb000000ffff },
>>>> +
>>>> +    /* 0xe02b - Ring 3 data */
>>>> +    [SEL2GDT(FLAT_RING3_DS32)] = { 0x00cff3000000ffff },
>>>> +
>>>> +    /* 0xe033 - Ring 3 code, 64-bit mode */
>>>> +    [SEL2GDT(FLAT_RING3_CS64)] = { 0x00affb000000ffff },
>>>> +
>>>> +    /* 0xe038 - Ring 0 code, compatibility */
>>>> +    [SEL2GDT(__HYPERVISOR_CS32)] = { 0x00cf9b000000ffff },
>>>> +
>>>> +    /* 0xe040 - TSS */
>>>> +    /* 0xe050 - LDT */
>>>> +
>>>> +    /* 0xe060 - per-CPU entry (limit == cpu) */
>>>> +    [SEL2GDT(PER_CPU_GDT_SELECTOR)] = { 0x0000910000000000 },
>>>
>>> It would be better if the = { } were vertically aligned.  It makes
>>> reading them easier.
>>>
>>> Also, now that SEL2GDT() is present, we need a BUILD_BUG_ON() to check
>>> that the size doesn't vary from one page.  At the moment, changing a
>>> selector to use 0xfxxx will cause this to grow beyond 1 page without any
>>> compiler diagnostic.  I'd go with
>>>
>>> static void __init __maybe_unused
>>> build_assertions(void)
>>>
>>> {
>>>       BUILD_BUG_ON(sizeof(boot_gdt) != PAGE_SIZE);
>>>       BUILD_BUG_ON(sizeof(boot_compat_gdt) != PAGE_SIZE);
>>> }
>>
>> Will do, albeit for the build assertions this isn't really the
>> right place imo, because this isn't the place where we depend
>> on them being just single pages. I'll put it there nevertheless,
>> but I'll add a comment for why they're there.
> 
> IMO this is the right place, because it is right beside where the array
> is specifically defined to be [PAGE_SIZE / sizeof()].

I was about to ask why we then need build_assertions() at all,
until I also saw ...

> What it is doing is working around what is arguably a compiler bug by
> allowing foo[x] = { [x + 1] = ... } to work.

... this. Which made me go check, and both gcc 4.3 and gcc 9.1
correctly complain "array index in initializer exceeds array
bounds".

> Anyway, with these assertions and the tweaked constant clenaup,
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

Thanks, but as per above I'm now irritated: With the explicit
specification of the array size, build_assertions() should either
be dropped again, or its comment be extended to cover why it's
needed _despite_ the specified array size (i.e. in which case
your example above would not cause a build failure).

Jan

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Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 10:34 [Xen-devel] [PATCH v2 0/2] x86/boot: cleanup Jan Beulich
2019-08-09 10:38 ` [Xen-devel] [PATCH v2 1/2] x86: define a few selector values Jan Beulich
2019-08-09 11:50   ` Andrew Cooper
2019-08-09 12:35     ` Jan Beulich
2019-08-23  2:38   ` Tian, Kevin
2019-08-09 10:40 ` [Xen-devel] [PATCH v2 2/2] x86/desc: Build boot_{, compat_}gdt[] in C Jan Beulich
2019-08-09 12:19   ` Andrew Cooper
2019-08-09 12:43     ` Jan Beulich
2019-08-09 13:07       ` Andrew Cooper
2019-08-09 13:18         ` Jan Beulich [this message]
2019-08-09 15:25           ` Andrew Cooper
2019-08-12  7:32   ` Jan Beulich
2019-08-12 10:36     ` Andrew Cooper
2019-08-09 10:41 ` [Xen-devel] [PATCH v2 0/2] x86/boot: cleanup Jan Beulich
2019-08-09 12:39 ` [Xen-devel] [PATCH 3/2] x86/desc: Drop __HYPERVISOR_CS32 Andrew Cooper
2019-08-09 12:50   ` Jan Beulich
2019-08-09 15:36     ` Andrew Cooper
2019-08-09 15:52       ` Jan Beulich

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