linux-kernel - Re: [PATCH] x86/speculation/l1tf: suggest what to do on systems with too much RAM

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Date:   Fri, 24 Aug 2018 14:10:54 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/speculation/l1tf: suggest what to do on systems with
 too much RAM

On 08/24/2018 12:36 PM, Vlastimil Babka wrote:
> On 08/24/2018 09:32 AM, Vlastimil Babka wrote:
>> On 08/23/2018 09:27 PM, Michal Hocko wrote:
>>> On Thu 23-08-18 16:28:12, Vlastimil Babka wrote:
>>>> Two users have reported [1] that they have an "extremely unlikely" system
>>>> with more than MAX_PA/2 memory and L1TF mitigation is not effective. Let's
>>>> make the warning more helpful by suggesting the proper mem=X kernel boot param,
>>>> a rough calculation of how much RAM can be lost (not precise if there's holes
>>>> between MAX_PA/2 and max_pfn in the e820 map) and a link to the L1TF document
>>>> to help decide if the mitigation is worth the unusable RAM.
>>>>
>>>> [1] https://bugzilla.suse.com/show_bug.cgi?id=1105536
>>>>
>>>> Suggested-by: Michal Hocko <mhocko@...e.com>
>>>> Cc: stable@...r.kernel.org
>>>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>>>
>>> I wouldn't bother with max_pfn-half_pa part but other than that this is
>>> much more useful than the original message.
>>
>> Right, and it causes build failures on some configs.
>>
>>> Acked-by: Michal Hocko <mhocko@...e.com>
>>
>> Thanks! Here's a v2:
> 
> Just realized that kvm printk's refer to the online version at
> https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html
> which should be easier for the users of distro kernels, should I change
> that?

FWIW, if it's not possible to amend anymore...
----8<----
>From a650e6a4b989c6e029ac1ab4e0a68553e074ba7a Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Fri, 24 Aug 2018 14:05:45 +0200
Subject: [PATCH] x86/speculation/l1tf: replace Documentation path with
 kernel.org URL

The URL might be easier to reach for users of distro kernels without unpacked
source, and be more up-to-date. It's also used in KVM warnings related to L1TF.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 arch/x86/kernel/cpu/bugs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 5c32b5006738..4c2313d0b9ca 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -705,7 +705,7 @@ static void __init l1tf_select_mitigation(void)
 		pr_info("You may make it effective by booting the kernel with mem=%llu parameter.\n",
 				half_pa);
 		pr_info("However, doing so will make a part of your RAM unusable.\n");
-		pr_info("Reading Documentation/admin-guide/l1tf.rst might help you decide.\n");
+		pr_info("Reading https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html might help you decide.\n");
 		return;
 	}
 
-- 
2.18.0