linux-kernel - Re: [PATCH Part2 v6 13/49] crypto:ccp: Provide APIs to issue SEV-SNP commands

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Date:   Tue, 2 Aug 2022 13:52:25 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     Ashish Kalra <Ashish.Kalra@....com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "Lendacky, Thomas" <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>, Marc Orr <marcorr@...gle.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Alper Gun <alpergun@...gle.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH Part2 v6 13/49] crypto:ccp: Provide APIs to issue SEV-SNP
 commands

On Tue, Jun 21, 2022 at 03:43:13PM -0600, Peter Gonda wrote:
> (
> 
> On Mon, Jun 20, 2022 at 5:05 PM Ashish Kalra <Ashish.Kalra@....com> wrote:
> >
> > From: Brijesh Singh <brijesh.singh@....com>
> >
> > Provide the APIs for the hypervisor to manage an SEV-SNP guest. The
> > commands for SEV-SNP is defined in the SEV-SNP firmware specification.
> >
> > Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> > ---
> >  drivers/crypto/ccp/sev-dev.c | 24 ++++++++++++
> >  include/linux/psp-sev.h      | 73 ++++++++++++++++++++++++++++++++++++
> >  2 files changed, 97 insertions(+)
> >
> > diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> > index f1173221d0b9..35d76333e120 100644
> > --- a/drivers/crypto/ccp/sev-dev.c
> > +++ b/drivers/crypto/ccp/sev-dev.c
> > @@ -1205,6 +1205,30 @@ int sev_guest_df_flush(int *error)
> >  }
> >  EXPORT_SYMBOL_GPL(sev_guest_df_flush);
> >
> > +int snp_guest_decommission(struct sev_data_snp_decommission *data, int *error)
> > +{
> > +       return sev_do_cmd(SEV_CMD_SNP_DECOMMISSION, data, error);
> > +}
> > +EXPORT_SYMBOL_GPL(snp_guest_decommission);
> > +
> > +int snp_guest_df_flush(int *error)
> > +{
> > +       return sev_do_cmd(SEV_CMD_SNP_DF_FLUSH, NULL, error);
> > +}
> > +EXPORT_SYMBOL_GPL(snp_guest_df_flush);

Nit: undocumented exported functions. Both need kdoc.

> 
> Why not instead change sev_guest_df_flush() to be SNP aware? That way
> callers get the right behavior without having to know if SNP is
> enabled or not.
> 
> int sev_guest_df_flush(int *error)
> {
>   if (!psp_master || !psp_master->sev_data)
>    return -EINVAL;
> 
>   if (psp_master->sev_data->snp_inited)
>     return sev_do_cmd(SEV_CMD_SNP_DF_FLUSH, NULL, error);
> 
>   return sev_do_cmd(SEV_CMD_DF_FLUSH, NULL, error);
> }

Because it serves no purpose to fuse them into one, and is only more
obfuscated (and also undocumented).

Two exported symbols can be traced also separately with ftrace/kprobes.

Degrading transparency is not great idea IMHO.

BR, Jarkko