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

[Michal Hocko]

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@xxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

I wouldn't bother with max_pfn-half_pa part but other than that this is
much more useful than the original message.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  arch/x86/kernel/cpu/bugs.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index cb4a16292aa7..4b820bb6c504 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -702,6 +702,11 @@ static void __init l1tf_select_mitigation(void)
>  	half_pa = (u64)l1tf_pfn_limit() << PAGE_SHIFT;
>  	if (e820__mapped_any(half_pa, ULLONG_MAX - half_pa, E820_TYPE_RAM)) {
>  		pr_warn("System has more than MAX_PA/2 memory. L1TF mitigation not effective.\n");
> +		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 up to %llu bytes of RAM unusable.\n",
> +				((u64) max_pfn << PAGE_SHIFT) - half_pa);
> +		pr_info("Reading Documentation/admin-guide/l1tf.rst might help you decide.");
>  		return;
>  	}
>  
> -- 
> 2.18.0

-- 
Michal Hocko
SUSE Labs