linux-kernel - Re: [PATCH v5 05/16] x86/cpu: Defer CR pinning setup until after EFI initialization

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Message-ID: <2e1c3685-6e6b-488d-ad8e-268193b13739@zytor.com>
Date: Tue, 29 Oct 2024 16:41:33 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: "Luck, Tony" <tony.luck@...el.com>,
        "Hansen, Dave"
 <dave.hansen@...el.com>,
        "Mehta, Sohil" <sohil.mehta@...el.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org"
 <x86@...nel.org>,
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        Ard Biesheuvel <ardb@...nel.org>,
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        Josh Poimboeuf <jpoimboe@...nel.org>,
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        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Brijesh Singh <brijesh.singh@....com>,
        Michael Roth <michael.roth@....com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
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Cc: Jonathan Corbet <corbet@....net>, Ingo Molnar <mingo@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
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Subject: Re: [PATCH v5 05/16] x86/cpu: Defer CR pinning setup until after EFI
 initialization

On 10/29/24 16:18, Luck, Tony wrote:
>>> Yeah, I was talking about 64-bit only.  On 32-bit PAE a PGD maps 1/4 of
>>> the address space which is totally unworkable for stealing.
>>
>> But it is also not necessary.
> 
> So maybe we could make the 64-bit version of use_temporary_mm()
> use some reserved address mapping to a reserved PGD in the upper
> half of address space, and the 32-bit version continue to use "user"
> addresses. It's unclear to me whether adding complexity here would be
> worth it to remove the 64-bit STAC/CLAC text patching issues.
> 

For 32 bits we can also simply use something further down in the 
hierarchy. It's not like we can afford to have the PGD be anything other 
than RWX on 32 bits.

	-hpa