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

[Andy Lutomirski]

On Thu, Oct 30, 2025, at 9:44 AM, Sohil Mehta 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.
>
> My primary goal here is to get the base LASS series merged (soonish)
> with the simplest possible option.
>
> I am planning to follow-up immediately with a vsyscall specific series
> that relaxes *most* restrictions.
>
> IIUC, supporting XONLY mode with LASS probably does not need complicated
> decoding because the vsyscall address is available in the faulting RIP.
>
> The spec says:
> "LASS for instruction fetches applies when the linear address in RIP is
> used to load an instruction from memory. Unlike canonicality checking
> (see Section 4.5.2), LASS does not apply to branch instructions that
> load RIP. A branch instruction can load RIP with an address that would
> violate LASS. Only when the address is used to fetch an instruction will
> a LASS violation occur, generating a #GP. (The return instruction
> pointer of the #GP handler is the address that incurred the LASS
> violation.)"
>
> I attempted to do that in the last revision here:
> https://lore.kernel.org/lkml/20251007065119.148605-9-sohil.mehta@xxxxxxxxx/
> https://lore.kernel.org/lkml/20251007065119.148605-11-sohil.mehta@xxxxxxxxx/
>
> On the other hand, supporting EMULATE mode during a #GP is a bit tricky,
> which isn't worth the effort.

I would say it's definitely worth the effort, but it probably does make sense to get the rest of the series in a mergeable condition such that it only works with vsyscall=none.