Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Date:   Thu, 26 May 2022 21:00:38 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        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,
        thomas.lendacky@amd.com, x86@kernel.org
Subject: Re: [PATCHv3 3/3] x86/tdx: Handle load_unaligned_zeropad()
 page-cross to a shared page
Message-ID: <Yo/qduyX33XHFVjK@google.com>
References: <20220524221012.62332-1-kirill.shutemov@linux.intel.com>
 <20220524221012.62332-4-kirill.shutemov@linux.intel.com>
 <6350b927-4b3f-6f43-aa62-f8db19fa8d5b@intel.com>
 <20220526203619.gpyyl67ygk622e5g@black.fi.intel.com>
 <bb94beeb-061d-c795-7ed6-3c9d2c73191c@intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <bb94beeb-061d-c795-7ed6-3c9d2c73191c@intel.com>
Precedence: bulk

On Thu, May 26, 2022, Dave Hansen wrote:
> 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.

Yeah, I don't think it's worth exploding on unaligned accesses, it's specifically
page splits that are a mess and are an absolutely nightmare to handle.  E.g. for
VirtIO kicks, the data and page offset are completely ignored/irrelevant, so a
multi-byte write to any random byte in the page should work, even though it's all
kinds of goofy.