linux-kernel - AMD Memory encryption vs. kexec

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Date:   Mon, 27 Nov 2023 16:00:50 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Borislav Petkov <bp@...en8.de>,
        "Shutemov, Kirill" <kirill.shutemov@...el.com>,
        Ashish Kalra <ashish.kalra@....com>,
        Kai Huang <kai.huang@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: AMD Memory encryption vs. kexec

... actually cc'd the mailing lists and x86@ exploder on this one.
Please reply here.

---

There are two kexec-related wbinvd's:

One for the kexec boot CPU in relocate_kernel() which is driven by
CC_ATTR_HOST_MEM_ENCRYPT:

>         image->start = relocate_kernel((unsigned long)image->head,
>                                        (unsigned long)page_list,
>                                        image->start,
>                                        image->preserve_context,
>                                        cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT));

the other is for non-boot CPUs in stop_this_cpu():

>         if (c->extended_cpuid_level >= 0x8000001f && (cpuid_eax(0x8000001f) & BIT(0)))
>                 native_wbinvd();

By my reading, the CC_ATTR_HOST_MEM_ENCRYPT is basically a check for
whether the current kernel has enabled SME but not SEV while the
stop_this_cpu() site is driven purely by whether the hardware *supports*
SME.

The whole supposed reason stop_this_cpu() checks CPUID directly is that
the current kernel SME/SEV enabling might not match the _next_ kernel's
enabling choices.

So, why is a _current_ kernel check OK for relocate_kernel(), but not OK
for stop_this_cpu()?

It seems to me like both sites might need to use the
stop_this_cpu()-style "raw" hardware support checks.

Why do I care?  TDX potentially needs wbinvd at the same two spots.  It
would be nice to have a common cc_attr for both sites, but I need to
reconcile the apparently disparate AMD uses first.