Re: [PATCH] x86/mm: Rework wbinvd, hlt operation in stop_this_cpu()

[Dave Young <dyoung@redhat.com>]

On 01/17/18 at 05:47pm, Tom Lendacky wrote:
> On 1/17/2018 5:41 PM, Tom Lendacky wrote:
> > Some issues have been reported with the for loop in stop_this_cpu() that
> > issues the 'wbinvd; hlt' sequence.  Reverting this sequence to halt()
> > has been shown to resolve the issue.
> > 
> > However, the wbinvd is needed when running with SME.  The reason for the
> > wbinvd is to prevent cache flush races between encrypted and non-encrypted
> > entries that have the same physical address.  This can occur when
> > kexec'ing from memory encryption active to inactive or vice-versa.  The
> > important thing is to not have outside of kernel text memory references
> > (such as stack usage), so the usage of the native_*() functions is needed
> > since these expand as inline asm sequences.  So instead of reverting the
> > change, rework the sequence.
> > 
> > Move the wbinvd instruction outside of the for loop as native_wbinvd()
> > and make its execution conditional on X86_FEATURE_SME.  In the for loop,
> > change the asm 'wbinvd; hlt' sequence back to a halt sequence but use
> > the native_halt() call.
> > 
> > Cc: <stable@vger.kernel.org> # 4.14.x
> > Fixes: bba4ed011a52 ("x86/mm, kexec: Allow kexec to be used with SME")
> > Reported-by: Dave Young <dyoung@redhat.com>
> 
> Dave,
> 
> Can you test this and see if it resolves your issue?

It works for me, thank you for the patch!

Tested-by: Dave Young <dyoung@redhat.com>

> 
> Thanks,
> Tom
> 
> > Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> > ---
> >  arch/x86/kernel/process.c |   25 +++++++++++++++----------
> >  1 file changed, 15 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index 63711fe..03408b9 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -379,19 +379,24 @@ void stop_this_cpu(void *dummy)
> >  	disable_local_APIC();
> >  	mcheck_cpu_clear(this_cpu_ptr(&cpu_info));
> >  
> > +	/*
> > +	 * 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();
> >  	for (;;) {
> >  		/*
> > -		 * Use wbinvd followed by hlt to stop the processor. This
> > -		 * provides support for kexec on a processor that supports
> > -		 * SME. With kexec, going from SME inactive to SME active
> > -		 * requires clearing cache entries so that addresses without
> > -		 * the encryption bit set don't corrupt the same physical
> > -		 * address that has the encryption bit set when caches are
> > -		 * flushed. To achieve this a wbinvd is performed followed by
> > -		 * a hlt. Even if the processor is not in the kexec/SME
> > -		 * scenario this only adds a wbinvd to a halting processor.
> > +		 * Use native_halt() so that memory contents don't change
> > +		 * (stack usage and variables) after possibly issuing the
> > +		 * native_wbinvd() above.
> >  		 */
> > -		asm volatile("wbinvd; hlt" : : : "memory");
> > +		native_halt();
> >  	}
> >  }
> >  
> >