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Date:   Tue, 24 Oct 2023 11:27:36 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
CC:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>, tony.luck@intel.com,
        ak@linux.intel.com, tim.c.chen@linux.intel.com,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        kvm@vger.kernel.org,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Daniel Sneddon <daniel.sneddon@linux.intel.com>,
        antonio.gomez.iglesias@linux.intel.com,
        Alyssa Milburn <alyssa.milburn@intel.com>
Subject: Re: [PATCH  v2 1/6] x86/bugs: Add asm helpers for executing VERW
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On October 24, 2023 10:02:48 AM PDT, Peter Zijlstra <peterz@infradead=2Eorg=
> wrote:
>On Tue, Oct 24, 2023 at 09:45:20AM -0700, Pawan Gupta wrote:
>
>> > > modules being within 4GB of kernel=2E
>
>FWIW, it's 2G, it's a s32 displacement, the highest most address can
>jump 2g down, while the lowest most address can jump 2g up=2E Leaving a 2=
G
>directly addressable range=2E
>
>And yeah, we ensure kernel text and modules are inside that 2G range=2E

To be specific, we don't require that it is located at any particular *phy=
sical* addresses, but all modules including the root module are remapped in=
to the [-2GiB,0) range=2E If we didn't do that, modules would have to be co=
mpiled with the pic memory model rather than the kernel memory model which =
is what they currently are=2E This would add substantial overhead due to th=
e need for a GOT (the PLT is optional if all symbols are resolved at load t=
ime=2E)

The kernel is different from user space objects since it is always fully l=
oaded into physical memory and is never paged or shared=2E Therefore, inlin=
e relocations, which break sharing and create dirty pages in user space, ha=
ve zero execution cost in the kernel; the only overhead to modules other th=
an load time (including the runtime linking) is that modules can't realisti=
cally be mapped using large page entries=2E