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

[Tom Lendacky]

On 2/4/22 12:00, Borislav Petkov wrote:
> 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...

Yep, and that should be safe. We would have to look at the generated code 
as there can't be any memory stores after the native_wbinvd() and before 
the native_halt().

Thanks,
Tom

>