Subject: Re: [PATCH 3/3] fs/core/vmcore: Move sev_active() reference to x86 arch code

From: Halil Pasic <[email-masked]>
Date: Mon, 15 Jul 2019 16:03:17 +0200

On Fri, 12 Jul 2019 18:55:47 -0300
Thiago Jung Bauermann <[email-masked]> wrote:

> 
> [ Cc'ing Tom Lendacky which I forgot to do earlier. Sorry about that. ]
> 
> Hello Halil,
> 
> Thanks for the quick review.
> 
> Halil Pasic <[email-masked]> writes:
> 
> > On Fri, 12 Jul 2019 02:36:31 -0300
> > Thiago Jung Bauermann <[email-masked]> wrote:
> >
> >> Secure Encrypted Virtualization is an x86-specific feature, so it shouldn't
> >> appear in generic kernel code because it forces non-x86 architectures to
> >> define the sev_active() function, which doesn't make a lot of sense.
> >
> > sev_active() might be just bad (too specific) name for a general
> > concept. s390 code defines it drives the right behavior in
> > kernel/dma/direct.c (which uses it).
> 
> I thought about that but couldn't put my finger on a general concept.
> Is it "guest with memory inaccessible to the host"?
> 

Well, force_dma_unencrypted() is a much better name thatn sev_active():
s390 has no AMD SEV, that is sure, but for virtio to work we do need to
make our dma accessible to the hypervisor. Yes, your "guest with memory
inaccessible to the host" shows into the right direction IMHO.
Unfortunately I don't have too many cycles to spend on this right now.

> Since your proposed definiton for force_dma_unencrypted() is simply to
> make it equivalent to sev_active(), I thought it was more
> straightforward to make each arch define force_dma_unencrypted()
> directly.

I did not mean to propose equivalence. I intended to say the name
sev_active() is not suitable for a common concept. On the other hand
we do have a common concept -- as common code needs to do or not do
things depending on whether "memory is protected/encrypted" or not. I'm
fine with the name force_dma_unencrypted(), especially because I don't
have a better name.

> 
> Also, does sev_active() drive the right behavior for s390 in
> elfcorehdr_read() as well?
> 

AFAIU, since s390 does not override it boils down to the same, whether
sev_active() returns true or false. I'm no expert in that area, but I
strongly hope that is the right behavior. @Janosch: can you help me
out with this one?

> >> To solve this problem, add an x86 elfcorehdr_read() function to override
> >> the generic weak implementation. To do that, it's necessary to make
> >> read_from_oldmem() public so that it can be used outside of vmcore.c.
> >>
> >> Signed-off-by: Thiago Jung Bauermann <[email-masked]>
> >> ---
> >>  arch/x86/kernel/crash_dump_64.c |  5 +++++
> >>  fs/proc/vmcore.c                |  8 ++++----
> >>  include/linux/crash_dump.h      | 14 ++++++++++++++
> >>  include/linux/mem_encrypt.h     |  1 -
> >>  4 files changed, 23 insertions(+), 5 deletions(-)
> >
> > Does not seem to apply to today's or yesterdays master.
> 
> It assumes the presence of the two patches I mentioned in the cover
> letter. Only one of them is in master.
> 
> I hadn't realized the s390 virtio patches were on their way to upstream.
> I was keeping an eye on the email thread but didn't see they were picked
> up in the s390 pull request. I'll add a new patch to this series making
> the corresponding changes to s390's <asm/mem_encrypt.h> as well.
> 

Being on cc for your patch made me realize that things got broken on
s390. Thanks! I've sent out a patch that fixes protvirt, but we are going
to benefit from your cleanups. I think with your cleanups and that patch
of mine both sev_active() and sme_active() can be removed. Feel free to
do so. If not, I can attend to it as well.

Regards,
Halil