linux-kernel - Re: [PATCH v6 6/6] drivers/node: Show in sysfs node's crypto capabilities

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Date:   Fri, 4 Feb 2022 19:00:49 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     "Limonciello, Mario" <mario.limonciello@....com>,
        Martin Fernandez <martin.fernandez@...ypsium.com>,
        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, linux-mm@...ck.org,
        tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, ardb@...nel.org,
        dvhart@...radead.org, andy@...radead.org,
        gregkh@...uxfoundation.org, rafael@...nel.org, rppt@...nel.org,
        akpm@...ux-foundation.org, daniel.gutson@...ypsium.com,
        hughsient@...il.com, alex.bazhaniuk@...ypsium.com,
        alison.schofield@...el.com, keescook@...omium.org
Subject: Re: [PATCH v6 6/6] drivers/node: Show in sysfs node's crypto
 capabilities

On Fri, Feb 04, 2022 at 11:12:04AM -0600, Tom Lendacky wrote:
> https://elixir.bootlin.com/linux/latest/source/arch/x86/kernel/process.c#L761

For those who won't open a browser just to see what he means :), that's
this snippet:

void stop_this_cpu(void *dummy):
	/*
	 * Use wbinvd on processors that support SME. This provides support
	 * for performing a successful kexec when going from SME inactive
	 * to SME active (or vice-versa). The cache must be cleared so that
	 * if there are entries with the same physical address, both with and
	 * without the encryption bit, they don't race each other when flushed
	 * and potentially end up with the wrong entry being committed to
	 * memory.
	 */
	if (boot_cpu_has(X86_FEATURE_SME))
		native_wbinvd();


Well, we do clear our *representation* of CPUID flags for other features
- see output of

$ git grep -E "(setup_)?clear_cpu_cap"

for examples. We do that for SME even: early_detect_mem_encrypt().

Which means, since this needs to be "processors that support SME", this
line should change to:

	/* ... test the CPUID bit directly because the machine might've cleared
	 * X86_FEATURE_SME due to cmdline options.
	 */
	if (cpuid_eax(0x8000001f) & BIT(0))
		native_wbinvd();

I'd say...

-- 
Regards/Gruss,
    Boris.

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