Re: [Xen-devel] [PATCH v2 2/2] x86/desc: Build boot_{, compat_}gdt[] in C

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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:
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>
>>> Introduce SEL2GDT(). Correct GDT indices in public header.
>>
>> "Correct" here is ambiguous because it implies there is a breakage.
>>
>> You appear to have reversed FLAT_RING3_CS64 and DS32 (retaining the
>> original comments) and changed the comments for FLAT_RING3_SS{32,64}.
>
> Well - the comments were what was wrong after all.
>
>> Except that now they are out of their logical order (CS 32 then 64, DS
>> 32 then 64, SS 32 then 64).
>>
>> What is the reasoning for the new order?  It isn't sorted by index.
>
> It is, as much as it looks to have been the original author's
> intent. I.e. I didn't re-order things across the nul selector
> between the two groups.I can pull FLAT_RING3_SS* up if that's
> what you want. I'm also happy with any other order that you
> may prefer - just let me know which one. All I care about is
> for the comments to be in sync with the actual values.

I think it would be clearest to leave the order as is, and just fix the
comments.

>
>>> --- /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()].

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

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

~Andrew

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