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
Date: Thu, 26 May 2022 21:00:38 +0000 [thread overview]
Message-ID: <Yo/qduyX33XHFVjK@google.com> (raw)
In-Reply-To: <bb94beeb-061d-c795-7ed6-3c9d2c73191c@intel.com>
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.
prev parent reply other threads:[~2022-05-26 21:00 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-24 22:10 [PATCHv3 0/3] Fix for load_unaligned_zeropad() in TDX guest Kirill A. Shutemov
2022-05-24 22:10 ` [PATCHv3 1/3] x86/tdx: Fix early #VE handling Kirill A. Shutemov
2022-05-24 22:10 ` [PATCHv3 2/3] x86/tdx: Clarify RIP adjustments in #VE handler Kirill A. Shutemov
2022-05-25 16:02 ` Dave Hansen
2022-05-26 20:13 ` Kirill A. Shutemov
2022-05-26 20:18 ` Dave Hansen
2022-05-24 22:10 ` [PATCHv3 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page Kirill A. Shutemov
2022-05-26 16:20 ` Dave Hansen
2022-05-26 20:36 ` Kirill A. Shutemov
2022-05-26 20:39 ` Dave Hansen
2022-05-26 21:00 ` Sean Christopherson [this message]
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