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

[Borislav Petkov]

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