Re: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging

[Dave Hansen]

On 3/10/22 06:32, David Laight wrote:
>> UAI allows software to store a tag in the upper 7 bits of a logical
>> address [63:57]. When enabled, the processor will suppress the
>> traditional canonical address checks on the addresses. More information
>> about UAI can be found in section 5.10 of 'AMD64 Architecture
>> Programmer's Manual, Vol 2: System Programming' which is available from
>>
>> https://bugzilla.kernel.org/attachment.cgi?id=300549
> Is that really allowing bit 63 to be used?
> That is normally the user-kernel bit.
> I can't help feeling that will just badly break things.

Yeah, this does seem worrisome.  The LAM approach[1] retains
canonicality checking for bit 63.


1.
https://www.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html