linux-kernel - Re: [PATCH 0/6] Introduce CET supervisor state support

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Message-ID: <7df3637c85517f5bc4e3583249f919c1b809f370.camel@intel.com>
Date: Thu, 11 Jul 2024 22:11:29 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Yang, Weijiang" <weijiang.yang@...el.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"x86@...nel.org" <x86@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>
CC: "john.allen@....com" <john.allen@....com>, "peterz@...radead.org"
	<peterz@...radead.org>, "Gao, Chao" <chao.gao@...el.com>,
	"mlevitsk@...hat.com" <mlevitsk@...hat.com>
Subject: Re: [PATCH 0/6] Introduce CET supervisor state support

On Thu, 2024-07-11 at 13:58 -0700, Dave Hansen wrote:
> So we're down to choosing between
> 
>  * $BYTES space in 'struct fpu' (on hardware supporting CET-S)
> 
> or
> 
>  * ~100 loc
> 
> $BYTES is 24, right?  Did I get anything wrong?

Do we know what the actual memory use is? It would increases the size asked of
of the allocator by 24 bytes, but what amount of memory actually gets reserved?

It is sometimes a slab allocated buffer, and sometimes a vmalloc, right? I'm not
sure about slab sizes, but for vmalloc if the increase doesn't cross a page
size, it will be the same size allocation in reality. Or if it is close to a
page size already, it might use a whole extra 4096 bytes.

So we might be looking at a situation where some tasks get an entire extra page
allocated per task, and some get no difference. And only the average is 24 bytes
increase.

Hmm, I was trying to reason out something to use for tie breaking. Not sure how
successful it was. The worst case of a whole page size sounds like something
someone might care about?