Re: [PATCH v11 9/9] x86/cpu: Enable LASS by default during CPU initialization

H. Peter Anvin
2025-10-31T06:42:47Z

On October 30, 2025 2:13:18 PM PDT, David Laight <david.laight.linux@gmail.com> wrote:
>On Thu, 30 Oct 2025 09:44:02 -0700
>Sohil Mehta <sohil.mehta@intel.com> wrote:
>
>> On 10/30/2025 8:45 AM, Andy Lutomirski wrote:
>> > On Thu, Oct 30, 2025, at 1:40 AM, H. Peter Anvin wrote:
>> >> Legacy vsyscalls have been obsolete for how long now?
>> >
>> > A looooong time.
>> >
>> > I would suggest defaulting LASS to on and *maybe* decoding just enough to log, once per boot, that a legacy vsyscall may have been attempted. It’s too bad that #GP doesn’t report the faulting address.
>> >
>>
>> Unfortunately, CONFIG_X86_VSYSCALL_EMULATION defaults to y. Also, the
>> default Vsyscall mode is XONLY. So even if vsyscalls are deprecated,
>> there is a non-zero possibility someone would complain about it.
>
>Presumably a command line parameter could be used to disable LASS
>in order to enable vsyscall emulation?
>
>That might let LASS be enabled by default.
>
> David
>

So I talked with Sohil about this earlier today, and there was a bit of a miscommunication — XONLY mode is just fine. It is read emulation mode that has problems.