linux-kernel - Re: [RFC v2-fix 1/1] x86/tdx: Handle in-kernel MMIO

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 18 May 2021 17:11:58 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2-fix 1/1] x86/tdx: Handle in-kernel MMIO

On Tue, May 18, 2021, Andi Kleen wrote:
> On 5/18/2021 8:00 AM, Dave Hansen wrote:
> > That sounds like something objective we can measure.  Does this cost 1
> > byte of extra text per readl/writel?  10?  100?
> 
> Alternatives are at least a pointer, but also the extra alternative code.
> It's definitely more than 10, I would guess 40+

The extra bytes for .altinstructions is very different than the extra bytes for
the code itself.  The .altinstructions section is freed after init, so yes it
bloats the kernel size a bit, but the runtime footprint is unaffected by the
patching metadata.

IIRC, patching read/write{b,w,l,q}() can be done with 3 bytes of .text overhead.

The other option to explore is to hook/patch IO_COND(), which can be done with
neglible overhead because the helpers that use IO_COND() are not inlined.  In a
TDX guest, redirecting IO_COND() to a paravirt helper would likely cover the
majority of IO/MMIO since virtio-pci exclusively uses the IO_COND() wrappers.
And if there are TDX VMMs that want to deploy virtio-mmio, hooking
drivers/virtio/virtio_mmio.c directly would be a viable option.