linux-kernel - Re: [PATCHv6 07/16] x86/vsyscall: Reorganize the #PF emulation code

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Message-ID: <D5C8ABB5-5185-43EC-8C3E-106DBBEBF389@zytor.com>
Date: Fri, 20 Jun 2025 20:35:45 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Dave Hansen <dave.hansen@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
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Subject: Re: [PATCHv6 07/16] x86/vsyscall: Reorganize the #PF emulation code

On June 20, 2025 4:21:38 PM PDT, Dave Hansen <dave.hansen@...el.com> wrote:
>On 6/20/25 16:08, Andrew Cooper wrote:
>>> But, the resulting code is wonky. It needs to do something more like this:
>>>
>>> 	if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
>>> 		return false;
>>>
>>> 	if (error_code & X86_PF_INSTR))
>>> 		return __emulate_vsyscall(regs, address);
>> To do this, LASS needs a proper interlink against NX || SMEP.
>> 
>> If neither NX nor SMEP are active, the CPU does not report X86_PF_INSTR,
>> meaning that fetches are reported as plain reads.
>Interesting point.
>
>I think the easiest way to do this is just make a cpuid_deps[] entry for
>LASS and NX. If there's a CPU where LASS is available but where NX isn't
>available, we have much bigger problems on our hands.

Indeed. There is exactly zero reason to support this case.