Re: [RFC PATCH 1/5] x86/kexec: Do unconditional WBINVD for bare-metal in stop_this_cpu()

[Tom Lendacky <thomas.lendacky@amd.com>]

On 3/14/25 11:28, Edgecombe, Rick P wrote:
> On Fri, 2025-03-14 at 10:11 -0500, Tom Lendacky wrote:
>>> I see that this already has Tom's RB, but I'm not sure how this works for
>>> AMD.
>>> The original SME patch tried to avoid writing to memory by putting the
>>> wbinvd
>>> immediately before the halt, but today it is further away. Below this hunk
>>> there
>>> are more instructions that could dirty memory before the halt.  Ohh... it's
>>> new.
>>> 9 months ago 26ba7353caaa ("x86/smp: Add smp_ops.stop_this_cpu() callback")
>>> adds
>>> a function call that would touch the stack. I think it's wrong? And probably
>>> introduced after this patch was originally written.
>>>
>>> Then the cpuid_eax() could be non-inlined, but probably not. But the
>>> boot_cpu_has() added in this patch could call out to kasan and dirty the
>>> stack.
>>>
>>> So I think the existing SME case might be theoretically incorrect, and if so
>>> this makes things very slightly worse.
>>
>> But the wbinvd() is performed after those checks, so everything gets flushed.
> 
> Oh, right, duh. Thanks for checking. Yea those shouldn't matter.
> 
> Does the stop_this_cpu() part never come into play for SME either? It looks like
> it was added for TDX guest kexec, but is a general ACPI thing.

It is a general ACPI thing, but I don't know of it being used by our MADT
tables.

> 
> Regarding the kasan thing, I was looking at this too:
> wbinvd()
> cpumask_clear_cpu()
>   clear_bit()
>     instrument_atomic_write()
>       kasan_check_write()
>         __kasan_check_write() <- non-inline

Yes, this does look worrisome. Too bad there isn't a way to turn off KASAN
for a single function.

Thanks,
Tom

>