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

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Message-ID: <286efdb9-7dbf-82f3-b172-29c575a3791b@intel.com>
Date:   Thu, 10 Mar 2022 08:45:44 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Bharata B Rao' <bharata@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "ananth.narayan@....com" <ananth.narayan@....com>
Subject: Re: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging

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