linux-kernel - RE: [PATCHv4 11/30] 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]

Message-ID: <45b6e0b6ceec46849754402c3da03fed@AcuMS.aculab.com>
Date:   Fri, 25 Feb 2022 02:23:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dave Hansen' <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "luto@...nel.org" <luto@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>
CC:     "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "knsathya@...nel.org" <knsathya@...nel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "sdeep@...are.com" <sdeep@...are.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "brijesh.singh@....com" <brijesh.singh@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCHv4 11/30] x86/tdx: Handle in-kernel MMIO

From: Dave Hansen
> Sent: 24 February 2022 20:12
...
> === Limitations of this approach ===
> 
> > MMIO addresses can be used with any CPU instruction that accesses
> > memory. Address only MMIO accesses done via io.h helpers, such as
> > 'readl()' or 'writeq()'.
> 
> Any CPU instruction that accesses memory can also be used to access
> MMIO.  However, by convention, MMIO access are typically performed via
> io.h helpers such as 'readl()' or 'writeq()'.
> 
> > readX()/writeX() helpers limit the range of instructions which can trigger
> > MMIO. It makes MMIO instruction emulation feasible. Raw access to a MMIO
> > region allows the compiler to generate whatever instruction it wants.
> > Supporting all possible instructions is a task of a different scope.
> 
> The io.h helpers intentionally use a limited set of instructions when
> accessing MMIO.  This known, limited set of instructions makes MMIO
> instruction decoding and emulation feasible in KVM hosts and SEV guests
> today.
> 
> MMIO accesses are performed without the io.h helpers are at the mercy of
> the compiler.  Compilers can and will generate a much more broad set of
> instructions which can not practically be decoded and emulated.  TDX
> guests will oops if they encounter one of these decoding failures.
> 
> This means that TDX guests *must* use the io.h helpers to access MMIO.
> 
> This requirement is not new.  Both KVM hosts and AMD SEV guests have the
> same limitations on MMIO access.

Am I reading the last sentence correctly?
Normally (on x86 at least) a driver can mmap() PCIe addresses directly
into a user process.
This lets a user process directly issue PCIe read/write bus cycles.
These can be any instructions at all.
I don't think we've had any issues doing that in normal VMs.

Or is this emulation only applying to specific PCIe slaves?

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)