Re: [PATCH v3 1/5] x86/kexec: do unconditional WBINVD for bare-metal in stop_this_cpu()

[Tom Lendacky]

On 4/10/24 17:26, Huang, Kai wrote:
> On 11/04/2024 4:14 am, Tom Lendacky wrote:
> > On 4/10/24 11:08, Tom Lendacky wrote:
> > > On 4/7/24 07:44, Kai Huang wrote:
> > >
> > > > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > > > index b8441147eb5e..5ba8a9c1e47a 100644
> > > > --- a/arch/x86/kernel/process.c
> > > > +++ b/arch/x86/kernel/process.c
> > > > @@ -813,18 +813,16 @@ void __noreturn stop_this_cpu(void *dummy)
> > > >       mcheck_cpu_clear(c);
> > > >       /*
> > > > -     * 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.
> > > > +     * The kernel could leave caches in incoherent state on SME/TDX
> > > > +     * capable platforms.  Flush cache to avoid silent memory
> > > > +     * corruption for these platforms.
> > > >        *
> > > > -     * Test the CPUID bit directly because the machine might've 
> > > > cleared
> > > > -     * X86_FEATURE_SME due to cmdline options.
> > > > +     * stop_this_cpu() is not a fast path, just do unconditional
> > > > +     * WBINVD for simplicity.  But only do WBINVD for bare-metal
> > > > +     * as TDX guests and SEV-ES/SEV-SNP guests will get unexpected
> > > > +     * (and unnecessary) #VE and may unable to handle.
> > >
> > > In addition to Kirill's comment on #VE...
> > >
> > > This last part of the comment reads a bit odd since you say 
> > > unconditional and then say only do WBINVD for bare-metal. Maybe 
> > > something like this makes it a bit clearer?:
> > >
> > > For TDX and SEV-ES/SEV-SNP guests, a WBINVD may cause an exception 
> > > (#VE or #VC). However, all exception handling has been torn down at 
> > > this point, so this would cause the guest to crash. Since memory 
> > > within these types of guests is coherent only issue the WBINVD on 
> > > bare-metal.
> >
> > Hmmm... actually it was the other WBINVD in patch #2 that caused the 
> > crash, so what I wrote above isn't accurate. You might want to re-word 
> > as appropriate.
>
> Yeah that's why I used "may unable to handle" in the comment, as I 
> thought there's no need to be that specific?

Yes, makes sense.

> I tend not to mention "memory within these types of guests is coherent". 
>   I mean the current upstream kernel _ONLY_ does WBINVD for SME, that 
> means for all non-SME environment there's no concern to do WBINVD here.
>
> Here we only extend to do WBINVD on more environments, so as long as 
> there's no harm to do WBINVD for them it's OK.
>
> How about below?
>
>      /*
>       * The kernel could leave caches in incoherent state on SME/TDX
>       * capable platforms.  Flush cache to avoid silent memory
>       * corruption for these platforms.
>       *
>       * For TDX and SEV-ES/SEV-SNP guests, a WBINVD causes an
>       * exception (#VE or #VC), and the kernel may not be able
>       * to handle such exception (e.g., TDX guest panics if it
>       * sees #VE).  Since stop_this_cpu() isn't a fast path, just
>       * issue the WBINVD on bare-metal instead of sprinkling
>       * around vendor-specific checks.
>       */

I think that's ok, but maybe just adding that the WBINVD is not 
necessary for TDX and SEV-ES/SEV-SNP would make it clearer. Just my 
opinion, though.

Thanks,
Tom

> > Thanks,
> > Tom
> >
> > > And you can expand the comment block out to at least 80 characters to 
> > > make it more compact.
>
> OK I can do.  I guess I have to change my vim setting to do so, though :-)