linux-kernel - Re: [RFC v2 28/32] x86/tdx: Make pages shared in ioremap()

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Date:   Fri, 21 May 2021 17:18:06 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFC v2 28/32] x86/tdx: Make pages shared in ioremap()

On Thu, May 20, 2021 at 01:12:58PM -0700, Kuppuswamy, Sathyanarayanan wrote:
> I see many variants of SEV/SME related checks in the common code path
> between TDX and SEV/SME. Can a generic call like
> protected_guest_has(MEMORY_ENCRYPTION) or is_protected_guest()
> replace all these variants?

It depends...

> We will not be able to test AMD related features. So I need to confirm
> it with AMD code maintainers/developers before making this change.

Lemme add two to Cc.

So looking at those examples, you guys are making it not very
suspenceful for TDX - it is the same function in all. :)

> arch/x86/include/asm/io.h:313:	if (sev_key_active() || is_tdx_guest()) {			\
> arch/x86/include/asm/io.h:329:	if (sev_key_active() || is_tdx_guest()) {			\

So I think the static key on the AMD side is not really needed and it
could be replaced with

	sev_active() && !sev_es_active()

i.e. SEV but but not SEV-ES. A vendor-agnostic function would do here
probably something like:

	protected_guest_has(ENC_UNROLL_STRING_IO)

and inside it, it would do:

	if (AMD)
		amd_protected_guest_has(...)
	else if (Intel)
		intel_protected_guest_has(...)
	else
		WARN()

and both vendors would each implement that function with the respective
low-level query functions.

> arch/x86/kernel/pci-swiotlb.c:52:	if (sme_active() || is_tdx_guest())

That can be probably

	protected_guest_has(ENC_HOST_MEM_ENCRYPT);

as on AMD that means SME but not SEV. I guess on Intel you guys want to
do bounce buffers in the guest? or so...

> arch/x86/mm/ioremap.c:96:	if (!sev_active() && !is_tdx_guest())

So that function should simply be replaced with:

        if (!(desc->flags & IORES_MAP_ENCRYPTED)) {
		/* ... comment bla explaining what this is... */
		if ((sev_active() || is_tdx_guest()) &&
		    (res->desc != IORES_DESC_NONE &&
		     res->desc != IORES_DESC_RESERVED))
				desc->flags |= IORES_MAP_ENCRYPTED;
	}

as to the first check I guess:

	protected_guest_has(ENC_GUEST_ENABLED)

or so to mean, kernel is running as an encrypted guest...

> arch/x86/mm/pat/set_memory.c:1984:	if (!mem_encrypt_active() && !is_tdx_guest())

That should probably be

	protected_guest_has(ENC_ACTIVE);

to denote the generic "I'm running some sort of memory encryption..."

Yeah, this is all rough and should show the main idea - to have a
vendor-agnostic accessor in such common code paths and then abstract
away the differences in cpu/amd.c and cpu/intel.c, respectively and thus
keep the code sane.

How does that sound?

ENC_ being an ENCryption prefix, ofc.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette