linux-kernel - Re: [PATCH 3/3] /dev/mem: Do not map unaccepted memory

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Date:   Mon, 11 Sep 2023 10:09:44 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
        Dave Young <dyoung@...hat.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-coco@...ts.linux.dev, linux-efi@...r.kernel.org,
        kexec@...ts.infradead.org
Subject: Re: [PATCH 3/3] /dev/mem: Do not map unaccepted memory

On 07.09.23 16:46, Dave Hansen wrote:
> On 9/7/23 07:25, Kirill A. Shutemov wrote:
>> On Thu, Sep 07, 2023 at 07:15:21AM -0700, Dave Hansen wrote:
>>> On 9/6/23 00:39, Adrian Hunter wrote:
>>>> Support for unaccepted memory was added recently, refer commit
>>>> dcdfdd40fa82 ("mm: Add support for unaccepted memory"), whereby
>>>> a virtual machine may need to accept memory before it can be used.
>>>>
>>>> Do not map unaccepted memory because it can cause the guest to fail.
>>> Doesn't /dev/mem already provide a billion ways for someone to shoot
>>> themselves in the foot?  TDX seems to have added the 1,000,000,001st.
>>> Is this really worth patching?
>> Is it better to let TD die silently? I don't think so.
> 
> First, let's take a look at all of the distro kernels that folks will
> run under TDX.  Do they have STRICT_DEVMEM set?

For virtio-mem, we do

	config VIRTIO_MEM
		...
		depends on EXCLUSIVE_SYSTEM_RAM

Which in turn:

	config EXCLUSIVE_SYSTEM_RAM
		...
		depends on !DEVMEM || STRICT_DEVMEM


Not supported on all archs, but at least on RHEL9 on x86_64 and aarch64.

So, making unaccepted memory similarly depend on "!DEVMEM || 
STRICT_DEVMEM" does not sound too far off ...


-- 
Cheers,

David / dhildenb