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

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Message-ID: <20230907100614.35mxxo63xwkz7ohw@box.shutemov.name>
Date:   Thu, 7 Sep 2023 13:06:14 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        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 Wed, Sep 06, 2023 at 10:39:02AM +0300, 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.
> 
> For /dev/mem, this means a read of unaccepted memory will return zeros,
> a write to unaccepted memory will be ignored, but an mmap of unaccepted
> memory will return an error.

I am unsure who currently uses /dev/mem. The change to the mmap path has the
potential to cause issues as it is a new behavior. However, it appears to
be a common practice as we also fail to mmap if PAT is set on a page in
the rang. I suppose it is acceptable.

Another option is to accept the memory on mmap, but it seems excessive at
this point.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov