linux-kernel - Re: [PATCHv3 3/3] x86/tdx: Handle load_unaligned

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Date:   Thu, 26 May 2022 13:39:37 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org, ak@...ux.intel.com,
        dan.j.williams@...el.com, david@...hat.com, hpa@...or.com,
        linux-kernel@...r.kernel.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com,
        thomas.lendacky@....com, x86@...nel.org
Subject: Re: [PATCHv3 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross
 to a shared page

On 5/26/22 13:36, Kirill A. Shutemov wrote:
> On Thu, May 26, 2022 at 09:20:56AM -0700, Dave Hansen wrote:
>> On 5/24/22 15:10, Kirill A. Shutemov wrote:
>>> +	/*
>>> +	 * MMIO accesses suppose to be naturally aligned and therefore never
>>> +	 * cross a page boundary. Seeing unaligned accesses indicates a bug or
>>> +	 * load_unaligned_zeropad() that steps into unmapped shared page.
>> Wait a sec though...
>>
>> We've been talking all along about how MMIO accesses are in some cases
>> just plain old compiler-generated memory accesses.  It's *probably* bad
>> code that does this, but it's not necessarily a bug.
> Compiler-generated memory accesses tend to be aligned too. You need to do
> something make them unalinged, like __packed or pointer trickery.

I totally agree.  But, the point is that __packed or pointer trickery is
goofy, but it's not necessarily a bug.  This might crash the kernel on
goofy stuff, not bugs.