Re: [PATCH v5 31/32] x86: Add sysfs support for Secure Memory Encryption

[Tom Lendacky]

On 4/27/2017 10:52 AM, Dave Hansen wrote:
> On 04/27/2017 12:25 AM, Dave Young wrote:
> > On 04/21/17 at 02:55pm, Dave Hansen wrote:
> > > On 04/18/2017 02:22 PM, Tom Lendacky wrote:
> > > > Add sysfs support for SME so that user-space utilities (kdump, etc.) can
> > > > determine if SME is active.
> > > >
> > > > A new directory will be created:
> > > >   /sys/kernel/mm/sme/
> > > >
> > > > And two entries within the new directory:
> > > >   /sys/kernel/mm/sme/active
> > > >   /sys/kernel/mm/sme/encryption_mask
> > >
> > > Why do they care, and what will they be doing with this information?
> >
> > Since kdump will copy old memory but need this to know if the old memory
> > was encrypted or not. With this sysfs file we can know the previous SME
> > status and pass to kdump kernel as like a kernel param.
> >
> > Tom, have you got chance to try if it works or not?
>
> What will the kdump kernel do with it though?  We kexec() into that
> kernel so the SME keys will all be the same, right?  So, will the kdump

Yes, the SME key will be same after a kexec.

> kernel be just setting the encryption bit in the PTE so it can copy the
> old plaintext out?

Yes, the idea would be to set the encryption bit in the PTE when mapping
and copying encrypted pages and not set it for unencrypted pages.

> Why do we need both 'active' and 'encryption_mask'?  How could it be
> that the hardware-enumerated 'encryption_mask' changes across a kexec()?

We don't need both, I just added the 'encryption mask' entry for
information. It won't change across a kexec, I can remove it.

Thanks,
Tom