linux-kernel - Re: [PATCH 5/7] x86/cpu: Enable LASS (Linear Address Space Separation)

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Date:   Thu, 12 Jan 2023 17:17:53 -0800
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Yian Chen <yian.chen@...el.com>,
        <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Paul Lai <paul.c.lai@...el.com>
Subject: Re: [PATCH 5/7] x86/cpu: Enable LASS (Linear Address Space
 Separation)

On 1/12/2023 10:17 AM, Dave Hansen wrote:

> In the end, want X86_CR4_LASS set when the kernel wants LASS and clear
> in *ALL* other cases.  That would be simply:
> 
> 	if (cpu_feature_enabled(X86_FEATURE_LASS)) {
> 		cr4_set_bits(X86_CR4_LASS);
> 	} else {
> 		cr4_clear_bits(X86_CR4_LASS);
> 	}
> 

Thanks for the explanation. This is very helpful.

> I'm also wondering if we even want a Kconfig option.  Is anyone
> realistically going to be compiling this support out?

I was initially thinking we should leave the Kconfig option for users
that really *need* vsyscall support. But, thinking about it more, we
don't need an LASS Kconfig option for that.

We can make CONFIG_LEGACY_VSYSCALL_NONE as the default instead of the
current CONFIG_LEGACY_VSYSCALL_XONLY. The kernel can disable LASS at
boot time if a user/admin has shown explicit preference by selecting
CONFIG_LEGACY_VSYSCALL_XONLY or by providing command line parameter
vsyscall=xonly/emulate.

Thoughts?