linux-kernel - Re: [PATCHv5 25/30] x86/tdx: Make pages shared in ioremap()

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Date:   Tue, 8 Mar 2022 14:02:54 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5 25/30] x86/tdx: Make pages shared in ioremap()

On 3/2/22 06:28, Kirill A. Shutemov wrote:
> In TDX guests, guest memory is protected from host access. If a guest
> performs I/O, it needs to explicitly share the I/O memory with the host.
> 
> Make all ioremap()ed pages that are not backed by normal memory
> (IORES_DESC_NONE or IORES_DESC_RESERVED) mapped as shared.
> 
> Since TDX memory encryption support is similar to AMD SEV architecture,
> reuse the infrastructure from AMD SEV code.
> 
> Co-developed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
> Reviewed-by: Tony Luck <tony.luck@...el.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/mm/ioremap.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
> index 026031b3b782..a5d4ec1afca2 100644
> --- a/arch/x86/mm/ioremap.c
> +++ b/arch/x86/mm/ioremap.c
> @@ -242,10 +242,15 @@ __ioremap_caller(resource_size_t phys_addr, unsigned long size,
>  	 * If the page being mapped is in memory and SEV is active then
>  	 * make sure the memory encryption attribute is enabled in the
>  	 * resulting mapping.
> +	 * In TDX guests, memory is marked private by default. If encryption
> +	 * is not requested (using encrypted), explicitly set decrypt
> +	 * attribute in all IOREMAPPED memory.
>  	 */

Nit: in this context, nobody knows what "private" means.

I'd probably just say this in the changelog:

	The permissions in PAGE_KERNEL_IO already work for "decrypted"
	memory on AMD SEV/SME systems.  That means that they have no
	need to make a pgprot_decrypted() call.

	TDX guests, on the other hand, _need_ change to PAGE_KERNEL_IO
	for "decrypted" mappings.  Add a pgprot_decrypted() for TDX.

I'm not sure you need a code comment.  There's really nothing that
mentions TDX in the code being commented.  If it needs clarification,
I'd do it behind the pgprot*() helpers.