Re: [PATCH Part2 v6 02/49] iommu/amd: Introduce function to check SEV-SNP support

Jarkko Sakkinen
2022-08-25T01:30:47Z

On Thu, Aug 25, 2022 at 04:28:12AM +0300, jarkko@kernel.org wrote:
> On Tue, Jun 21, 2022 at 11:50:59AM -0600, Peter Gonda wrote:
> > On Tue, Jun 21, 2022 at 11:45 AM Kalra, Ashish <Ashish.Kalra@amd.com> wrote:
> > >
> > > [AMD Official Use Only - General]
> > >
> > > Hello Peter,
> > >
> > > >> +bool iommu_sev_snp_supported(void)
> > > >> +{
> > > >> + struct amd_iommu *iommu;
> > > >> +
> > > >> + /*
> > > >> + * The SEV-SNP support requires that IOMMU must be enabled, and is
> > > >> + * not configured in the passthrough mode.
> > > >> + */
> > > >> + if (no_iommu || iommu_default_passthrough()) {
> > > >> + pr_err("SEV-SNP: IOMMU is either disabled or
> > > >> + configured in passthrough mode.\n");
> > >
> > > > Like below could this say something like snp support is disabled because of iommu settings.
> > >
> > > Here we may need to be more precise with the error information indicating why SNP is not enabled.
> > > Please note that this patch may actually become part of the IOMMU + SNP patch series, where
> > > additional checks are done, for example, not enabling SNP if IOMMU v2 page tables are enabled,
> > > so precise error information will be useful here.
> >
> > I agree we should be more precise. I just thought we should explicitly
> > state something like: "SEV-SNP: IOMMU is either disabled or configured
> > in passthrough mode, SNP cannot be supported".
>
> It really should be, in order to have any practical use:
>
> if (no_iommu) {
> pr_err("SEV-SNP: IOMMU is disabled.\n");
> return false;
> }
>
> if (iommu_default_passthrough()) {
> pr_err("SEV-SNP: IOMMU is configured in passthrough mode.\n");
> return false;
> }
>
> The comment is *completely* redundant, it absolutely does
> not serve any sane purpose. It just tells what the code
> already clearly stating.
>
> The combo error message on the other hand leaves you to
> the question "which one was it", and for that reason
> combining the checks leaves you to a louse debugging
> experience.

Also, are those really *errors*? That implies that there
is something wrong.

Since you can have a legit configuration, IMHO they should
be either warn or info. What do you think?

They are definitely not errors.

BR, Jarkko