Messages in this thread | | | Date | Thu, 26 May 2022 21:00:38 +0000 | From | Sean Christopherson <> | Subject | Re: [PATCHv3 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page |
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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.
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