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Message-ID: <bb94beeb-061d-c795-7ed6-3c9d2c73191c@intel.com>
Date:   Thu, 26 May 2022 13:39:37 -0700
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Subject: Re: [PATCHv3 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross
 to a shared page
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        luto@kernel.org, peterz@infradead.org, ak@linux.intel.com,
        dan.j.williams@intel.com, david@redhat.com, hpa@zytor.com,
        linux-kernel@vger.kernel.org,
        sathyanarayanan.kuppuswamy@linux.intel.com, seanjc@google.com,
        thomas.lendacky@amd.com, x86@kernel.org
References: <20220524221012.62332-1-kirill.shutemov@linux.intel.com>
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 <20220526203619.gpyyl67ygk622e5g@black.fi.intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20220526203619.gpyyl67ygk622e5g@black.fi.intel.com>
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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.