linux-kernel - Re: [PATCH 1/2] x86/build: only align ENTRY_TEXT to PMD

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Message-ID: <e02db21f-c08a-408a-bfc0-c4feb628ce52@suse.com>
Date: Thu, 17 Jul 2025 11:32:59 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Dave Hansen <dave.hansen@...el.com>,
 Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>,
 linux-kernel@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, David Woodhouse <dwmw@...zon.co.uk>,
 Guenter Roeck <linux@...ck-us.net>, Jared White <jaredwhite@...rosoft.com>
Subject: Re: [PATCH 1/2] x86/build: only align ENTRY_TEXT to PMD_SIZE if
 necessary

On 10.07.25 г. 16:14 ч., Dave Hansen wrote:
> On 7/9/25 13:16, Hamza Mahfooz wrote:
>> PTI requires the begin and end of ENTRY_TEXT be aligned to PMD_SIZE.
>> SRSO requires srso_alias_untrain_ret to be 2M aligned. This costs
>> between 2-4 MiB of RAM (depending on the size of the preceding section).
>> So, only align when PTI is enabled or SRSO is enabled.
> 
> This seems so utterly random.
> 
> I don't think I was even aware of the SRSO restriction here. Looking
> over it now, I do see the vmlinux.lds.S changes and this does make sense.
> 
> But I'm really worried that we've grown more dependencies on this
> alignment. Let's say, for instance, that you forgot to address SRSO in
> this patch and the mitigation got broken. Would we have ever known?

Yes, because there are some ASSERT in the linker file that ensure the 
various SRSO thunks have appropriate alignment.

> 
> I'd like to hear a lot more from you about why 2-4 MiB of RAM is
> important and what the environment is where you presumably know that
> there are no Meltdown or SRSO vulnerable CPUs.
>